[PATCH] mm: filemap: tighten dropbehind completion context check

[PATCH] mm: filemap: tighten dropbehind completion context check

[Wenjie Qi]

folio_end_dropbehind() uses in_task() to keep folio invalidation out of
interrupt context. Task context alone is not sufficient: preemption can
still be disabled, or the task can be in a preemptible RCU read-side
critical section, while filemap_end_dropbehind() may reach
folio_unmap_invalidate() and sleep.

Use the established conservative three-part atomic-context test: reject
preemptible RCU read-side sections, reject configurations without
PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
completions retain the existing best-effort behavior and skip invalidation.

Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
---
 mm/filemap.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec6368..0616057e6 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -45,6 +45,7 @@
 #include <linux/migrate.h>
 #include <linux/pipe_fs_i.h>
 #include <linux/splice.h>
+#include <linux/rcupdate.h>
 #include <linux/rcupdate_wait.h>
 #include <linux/sched/mm.h>
 #include <linux/sysctl.h>
@@ -1620,6 +1621,15 @@ static void filemap_end_dropbehind(struct folio *folio)
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
+static bool folio_dropbehind_in_atomic(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
+		return true;
+	if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
+		return true;
+	return !preemptible();
+}
+
 /*
  * If folio was marked as dropbehind, then pages should be dropped when writeback
  * completes. Do that now. If we fail, it's likely because of a big folio -
@@ -1631,13 +1641,12 @@ void folio_end_dropbehind(struct folio *folio)
 		return;
 
 	/*
-	 * Hitting !in_task() should not happen off RWF_DONTCACHE writeback,
-	 * but can happen if normal writeback just happens to find dirty folios
-	 * that were created as part of uncached writeback, and that writeback
-	 * would otherwise not need non-IRQ handling. Just skip the
+	 * Hitting an atomic context should not happen from RWF_DONTCACHE
+	 * writeback, but can happen if normal writeback just happens to find
+	 * dirty folios created as part of uncached writeback. Just skip the
 	 * invalidation in that case.
 	 */
-	if (in_task() && folio_trylock(folio)) {
+	if (!folio_dropbehind_in_atomic() && folio_trylock(folio)) {
 		filemap_end_dropbehind(folio);
 		folio_unlock(folio);
 	}
-- 
2.43.0

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Matthew Wilcox]

On Thu, Aug 20, 2026 at 10:29:56PM +0800, Wenjie Qi wrote:
> folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> interrupt context. Task context alone is not sufficient: preemption can
> still be disabled, or the task can be in a preemptible RCU read-side
> critical section, while filemap_end_dropbehind() may reach
> folio_unmap_invalidate() and sleep.
> 
> Use the established conservative three-part atomic-context test: reject
> preemptible RCU read-side sections, reject configurations without
> PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> completions retain the existing best-effort behavior and skip invalidation.

Have you seen this happen in practice, or is this based on code
examination?

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Wenjie Qi]

This is based on code examination; I have not reproduced it in
  folio_end_dropbehind().
  There is an analogous EROFS report where bio completion ran under an RCU
  read-side critical section and hit a sleeping-function warning even though
  in_atomic() and preempt_count were both zero:

  https://lore.kernel.org/r/20230621220848.3379029-1-dhavale@google.com

  That is not a reproducer for this path, but it shows why task context alone
  does not establish that sleeping is safe. Here, folio_unmap_invalidate() can
  reach unmap_mapping_folio(), which takes mapping->i_mmap_rwsem through
  i_mmap_lock_read(). I noticed the mismatch while comparing this path with
  the stricter bio_in_atomic() check used by the block dropbehind work.

On Thu, Aug 20, 2026 at 10:34 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, Aug 20, 2026 at 10:29:56PM +0800, Wenjie Qi wrote:
> > folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> > interrupt context. Task context alone is not sufficient: preemption can
> > still be disabled, or the task can be in a preemptible RCU read-side
> > critical section, while filemap_end_dropbehind() may reach
> > folio_unmap_invalidate() and sleep.
> >
> > Use the established conservative three-part atomic-context test: reject
> > preemptible RCU read-side sections, reject configurations without
> > PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> > completions retain the existing best-effort behavior and skip invalidation.
>
> Have you seen this happen in practice, or is this based on code
> examination?

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Matthew Wilcox]

On Fri, Aug 21, 2026 at 09:23:12AM +0800, Wenjie Qi wrote:
> This is based on code examination; I have not reproduced it in
>   folio_end_dropbehind().
>   There is an analogous EROFS report where bio completion ran under an RCU
>   read-side critical section and hit a sleeping-function warning even though
>   in_atomic() and preempt_count were both zero:
> 
>   https://lore.kernel.org/r/20230621220848.3379029-1-dhavale@google.com
> 
>   That is not a reproducer for this path, but it shows why task context alone
>   does not establish that sleeping is safe. Here, folio_unmap_invalidate() can
>   reach unmap_mapping_folio(), which takes mapping->i_mmap_rwsem through
>   i_mmap_lock_read(). I noticed the mismatch while comparing this path with
>   the stricter bio_in_atomic() check used by the block dropbehind work.

So your analysis is right as far as it goes.  But if a folio has
been marked as dropbehind, but was then mmaped, we clearly shouldn't
be discarding it!  I believe that we'll clear the dropbehind flag in
__filemap_get_folio_mpol(), called from filemap_get_folio() called from
filemap_fault().

If you can find a way to get a folio with both dropbehind & mapped set,
I'm interested in hearing how.

> On Thu, Aug 20, 2026 at 10:34 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Thu, Aug 20, 2026 at 10:29:56PM +0800, Wenjie Qi wrote:
> > > folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> > > interrupt context. Task context alone is not sufficient: preemption can
> > > still be disabled, or the task can be in a preemptible RCU read-side
> > > critical section, while filemap_end_dropbehind() may reach
> > > folio_unmap_invalidate() and sleep.
> > >
> > > Use the established conservative three-part atomic-context test: reject
> > > preemptible RCU read-side sections, reject configurations without
> > > PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> > > completions retain the existing best-effort behavior and skip invalidation.
> >
> > Have you seen this happen in practice, or is this based on code
> > examination?

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Wenjie Qi]

  I think fault-around provides such a path.

  do_read_fault() calls do_fault_around() before ->fault.  For generic file
  mappings this reaches filemap_map_pages(), which walks mapping->i_pages
  directly and maps ready folios without going through
  __filemap_get_folio_mpol().  It does not clear dropbehind or exclude dirty
  or writeback folios.

  If the mapped PTE range covers vmf->address, filemap_map_pages() returns
  VM_FAULT_NOPAGE, so do_read_fault() does not fall back to filemap_fault().
  Both the faulting folio and speculative neighboring folios can therefore be
  mapped while retaining dropbehind.

  A fault-around PTE does not necessarily mean that every mapped folio was
  accessed, since neighboring folios are mapped speculatively.  So I do not
  think folio_mapped() alone is sufficient to cancel dropbehind.  Clearing it
  for the folio covering vmf->address may be more precise, but that would be a
  separate semantic change.

  If this distinction makes sense, I can send an RFC to clear dropbehind for
  the faulting folio in filemap_map_pages().

On Fri, Aug 21, 2026 at 11:30 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Aug 21, 2026 at 09:23:12AM +0800, Wenjie Qi wrote:
> > This is based on code examination; I have not reproduced it in
> >   folio_end_dropbehind().
> >   There is an analogous EROFS report where bio completion ran under an RCU
> >   read-side critical section and hit a sleeping-function warning even though
> >   in_atomic() and preempt_count were both zero:
> >
> >   https://lore.kernel.org/r/20230621220848.3379029-1-dhavale@google.com
> >
> >   That is not a reproducer for this path, but it shows why task context alone
> >   does not establish that sleeping is safe. Here, folio_unmap_invalidate() can
> >   reach unmap_mapping_folio(), which takes mapping->i_mmap_rwsem through
> >   i_mmap_lock_read(). I noticed the mismatch while comparing this path with
> >   the stricter bio_in_atomic() check used by the block dropbehind work.
>
> So your analysis is right as far as it goes.  But if a folio has
> been marked as dropbehind, but was then mmaped, we clearly shouldn't
> be discarding it!  I believe that we'll clear the dropbehind flag in
> __filemap_get_folio_mpol(), called from filemap_get_folio() called from
> filemap_fault().
>
> If you can find a way to get a folio with both dropbehind & mapped set,
> I'm interested in hearing how.
>
> > On Thu, Aug 20, 2026 at 10:34 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > On Thu, Aug 20, 2026 at 10:29:56PM +0800, Wenjie Qi wrote:
> > > > folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> > > > interrupt context. Task context alone is not sufficient: preemption can
> > > > still be disabled, or the task can be in a preemptible RCU read-side
> > > > critical section, while filemap_end_dropbehind() may reach
> > > > folio_unmap_invalidate() and sleep.
> > > >
> > > > Use the established conservative three-part atomic-context test: reject
> > > > preemptible RCU read-side sections, reject configurations without
> > > > PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> > > > completions retain the existing best-effort behavior and skip invalidation.
> > >
> > > Have you seen this happen in practice, or is this based on code
> > > examination?

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Andrew Morton]

On Thu, 20 Aug 2026 22:29:56 +0800 Wenjie Qi <qwjhust@gmail.com> wrote:

> folio_end_dropbehind() uses in_task() to keep folio invalidation out of
> interrupt context. Task context alone is not sufficient: preemption can
> still be disabled, or the task can be in a preemptible RCU read-side
> critical section, while filemap_end_dropbehind() may reach
> folio_unmap_invalidate() and sleep.
> 
> Use the established conservative three-part atomic-context test: reject
> preemptible RCU read-side sections, reject configurations without
> PREEMPT_COUNT, and otherwise require a preemptible context. Unsafe
> completions retain the existing best-effort behavior and skip invalidation.
> 
> ...
>
> +static bool folio_dropbehind_in_atomic(void)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
> +		return true;
> +	if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
> +		return true;
> +	return !preemptible();
> +}

Cripes.

There's nothing mm-specific about this function.  If we have a use-case
for such a thing then surely the function should be kernel-wide, it
should live at the sched/rcu/etc layer and it should be elaborately
documented?

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Wenjie Qi]

I kept the helper private to avoid an MM dependency on <linux/bio.h>.

After looking further at the mapped-folio case Matthew raised, I think the
better fix is to retain a dropbehind folio once it is mapped.  A mapping
represents a competing cached user, and retaining it avoids the sleeping
unmap path, so filemap no longer needs a stricter context helper; the
existing in_task() guard remains.  I will respin the patch on that basis.

[PATCH v2] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

From: Wenjie Qi <qiwenjie@xiaomi.com>

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind. A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping it
when writeback completes.

Unmapped dropbehind folios continue through the existing invalidation path.

Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
---
Changes since v1:
- Retain mapped folios instead of adding a stricter context predicate.
- Remove the private helper and RCU include.

 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..00fd89cf6f55 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
-- 
2.43.0

Re: [PATCH v2] mm: filemap: retain mapped dropbehind folios

[Matthew Wilcox]

On Sat, Aug 29, 2026 at 08:30:47PM +0800, Wenjie Qi wrote:
> From: Wenjie Qi <qiwenjie@xiaomi.com>
> 
> Fault-around can map ready dropbehind folios without going through the
> normal page-cache lookup that clears dropbehind. A mapping represents a
> competing cached user, so retain the folio instead of forcibly unmapping it
> when writeback completes.
> 
> Unmapped dropbehind folios continue through the existing invalidation path.

Yes, I think this is the right approach.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Re: [PATCH v2] mm: filemap: retain mapped dropbehind folios

[Tal Zussman]

On Sat, 29 Aug 2026 20:30:47 +0800, Wenjie Qi <qwjhust@gmail.com> wrote:
> Fault-around can map ready dropbehind folios without going through the
> normal page-cache lookup that clears dropbehind. A mapping represents a
> competing cached user, so retain the folio instead of forcibly unmapping it
> when writeback completes.
> 
> Unmapped dropbehind folios continue through the existing invalidation path.
> 
> Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>

Overall looks good to me. It may be nice to add a sentence about the
safety aspect of this in the commit message. Maybe something like:

  folio_unmap_invalidate() may sleep if the folio is mapped, so this
  also ensures that folio_end_dropbehind() is safe to call from
  non-preemptible task context.

Reviewed-by: Tal Zussman <tz2294@columbia.edu>

-- 
Tal Zussman <tz2294@columbia.edu>

Re: [PATCH v2] mm: filemap: retain mapped dropbehind folios

[Tal Zussman]

On 8/29/26 6:53 PM, Tal Zussman wrote:
> On Sat, 29 Aug 2026 20:30:47 +0800, Wenjie Qi <qwjhust@gmail.com> wrote:
>> Fault-around can map ready dropbehind folios without going through the
>> normal page-cache lookup that clears dropbehind. A mapping represents a
>> competing cached user, so retain the folio instead of forcibly unmapping it
>> when writeback completes.
>>
>> Unmapped dropbehind folios continue through the existing invalidation path.
>>
>> Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
> 
> Overall looks good to me. It may be nice to add a sentence about the
> safety aspect of this in the commit message. Maybe something like:
> 
>   folio_unmap_invalidate() may sleep if the folio is mapped, so this
>   also ensures that folio_end_dropbehind() is safe to call from
>   non-preemptible task context.
> 
> Reviewed-by: Tal Zussman <tz2294@columbia.edu>
> 

And maybe:

Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")

[PATCH v3] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

From: Wenjie Qi <qiwenjie@xiaomi.com>

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind. A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping it
when writeback completes.

For a mapped folio, folio_unmap_invalidate() can call
unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep. Retaining
mapped folios avoids this path when folio_end_dropbehind() runs in
non-preemptible task context.

Unmapped dropbehind folios continue through the existing invalidation path.

Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Tal Zussman <tz2294@columbia.edu>
---
Changes since v2:
- Explain why retaining mapped folios avoids a sleepable unmap path.
- Add the Fixes and Reviewed-by trailers.

 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..00fd89cf6f55 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
-- 
2.43.0

Re: [PATCH v3] mm: filemap: retain mapped dropbehind folios

[Andrew Morton]

On Sun, 30 Aug 2026 01:36:12 +0800 Wenjie Qi <qwjhust@gmail.com> wrote:

> From: Wenjie Qi <qiwenjie@xiaomi.com>
> 
> Fault-around can map ready dropbehind folios without going through the
> normal page-cache lookup that clears dropbehind. A mapping represents a
> competing cached user, so retain the folio instead of forcibly unmapping it
> when writeback completes.
> 
> For a mapped folio, folio_unmap_invalidate() can call
> unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep. Retaining
> mapped folios avoids this path when folio_end_dropbehind() runs in
> non-preemptible task context.
> 
> Unmapped dropbehind folios continue through the existing invalidation path.

Thanks.

When fixing a bug, please always provide a very clear description of the
userspace-visible runtime effects of that bug.

From the above it appears that the current code can trigger a
sleeping-in-atomic warning?  Has this been observed in any situation? 
Is there a report?

chatgpt easily prepared a reproducer for me, which I haven't run.

If I'm correct in the above, we should backport this fix with a
cc:stable tag, do you agree?

[PATCH v4] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

From: Wenjie Qi <qiwenjie@xiaomi.com>

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind. A mapping represents a
competing cached user, but writeback completion can currently unmap that
folio. A later mmap access must then fault it back in.

Retain mapped folios instead. For a mapped folio,
folio_unmap_invalidate() can call unmap_mapping_folio(), which takes
i_mmap_rwsem and may sleep. Retaining the folio also avoids this path when
folio_end_dropbehind() runs in non-preemptible task context.

Unmapped dropbehind folios continue through the existing invalidation path.

Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
Cc: stable@vger.kernel.org
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Tal Zussman <tz2294@columbia.edu>
---
The mapped-plus-dropbehind state was reproduced in QEMU. The patched
kernel retained the mapped folio and continued to evict the unmapped
dontcache folio. I did not reproduce the sleeping-in-atomic warning and
am not aware of an existing report.

Changes since v3:
- Describe the forced-unmap/refault runtime effect.
- Record the reproduction boundary and add Cc: stable@vger.kernel.org.

 mm/filemap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 6afec636881f..00fd89cf6f55 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
-- 
2.43.0

Re: [PATCH v4] mm: filemap: retain mapped dropbehind folios

[Barry Song]

On Sun, Aug 30, 2026 at 11:25 AM Wenjie Qi <qwjhust@gmail.com> wrote:
>
> From: Wenjie Qi <qiwenjie@xiaomi.com>
>
> Fault-around can map ready dropbehind folios without going through the
> normal page-cache lookup that clears dropbehind. A mapping represents a
> competing cached user, but writeback completion can currently unmap that
> folio. A later mmap access must then fault it back in.

Andrew is asking for a visible user-facing impact. I think the current
mainline code might result in an extra fault-in and extra I/O. Maybe we
should highlight this?
On the other hand, my gut feeling is that this patch might only be
50% right, rather than 100% right.
Fault-around might just end up mapping data that will never be accessed
at all (fault-around mappings aren't really fault-mapped), while the user
has explicitly requested "dropbehind", which seems like a strong hint
that they don't need it :-)

>
> Retain mapped folios instead. For a mapped folio,
> folio_unmap_invalidate() can call unmap_mapping_folio(), which takes
> i_mmap_rwsem and may sleep. Retaining the folio also avoids this path when
> folio_end_dropbehind() runs in non-preemptible task context.
>

I'm not quite sure this can be entirely avoided. Somehow,
`folio_launder(mapping, folio)` could also be nasty. For example, NFS
can still be problematic today:

static int nfs_launder_folio(struct folio *folio)
{
        struct inode *inode = folio->mapping->host;
        int ret;

        dfprintk(PAGECACHE, "NFS: launder_folio(%llu, %llu)\n",
                inode->i_ino, folio_pos(folio));

        folio_wait_private_2(folio); /* [DEPRECATED] */
        ret = nfs_wb_folio(inode, folio);
        trace_nfs_launder_folio_done(inode, folio_pos(folio),
                        folio_size(folio), ret);
        return ret;
}

Best Regards
Barry

Re: [PATCH] mm: filemap: tighten dropbehind completion context check

[Barry Song]

On Sat, Aug 29, 2026 at 8:13 PM Wenjie Qi <qwjhust@gmail.com> wrote:
>
> I kept the helper private to avoid an MM dependency on <linux/bio.h>.
>
> After looking further at the mapped-folio case Matthew raised, I think the
> better fix is to retain a dropbehind folio once it is mapped.  A mapping
> represents a competing cached user, and retaining it avoids the sleeping
> unmap path, so filemap no longer needs a stricter context helper; the
> existing in_task() guard remains.  I will respin the patch on that basis.

But we still have `bio_in_atomic()` in `include/linux/bio.h`:

/**
 * bio_in_atomic - check if the current context is unsafe for bio completion
 *
 * Return: %true in atomic contexts (e.g. hard/soft IRQ, preempt-disabled);
 * %false when a bio can be safely completed in the current context.
 */
static inline bool bio_in_atomic(void)
{
        if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
                return true;
        if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
                return true;
        return !preemptible();
}

Right now, it's used in three places, and I expect we'll see more users
over time.

   1   1925  block/bio.c <<bio_endio>>
             if (bio_flagged(bio, BIO_COMPLETE_IN_TASK) && bio_in_atomic())
   2   1451  fs/erofs/zdata.c <<z_erofs_decompress_kickoff>>
             if (bio_in_atomic()) {
   3    403  include/linux/bio.h <<bio_complete_in_task>>
             if (!bio_in_atomic())

Yet it really has nothing to do with `bio` at all; it's purely a
scheduler thing!

So I fully agree with Andrew that we should move it to `sched` and
continue the discussion there. Maybe the current `in_atomic()` helpers
aren't really good enough?

Best Regards
Barry

Re: [PATCH v4] mm: filemap: retain mapped dropbehind folios

[Wenjie Qi]

Yes.  The current path removes the folio from the page cache, so a later
mmap access can incur both another fault and I/O.  I should have stated
that explicitly.

For fault-around neighbors, folio_mapped() cannot distinguish the faulting
page from a speculative neighbor.  This patch takes the conservative policy
that an installed PTE represents a competing cache user and wins over the
writer's RWF_DONTCACHE hint.  It can therefore retain a speculative neighbor
which is never accessed.  Distinguishing those cases seems to require
fault-around to preserve the faulting folio while not mapping, or later
dropping, dropbehind neighbors.  I think that should be considered as a
separate follow-up.

For folio_launder(), filemap_end_dropbehind() holds the folio lock and returns
if the folio is dirty or under writeback before calling
folio_unmap_invalidate().  folio_launder() also immediately returns for a
clean folio.  I do not see how nfs_launder_folio() is reached from this
completion path unless the locked, unmapped folio can become dirty between
those checks.  Is there a path I am missing?

Re: [PATCH v4] mm: filemap: retain mapped dropbehind folios

[Barry Song]

On Sun, Aug 30, 2026 at 5:13 PM Wenjie Qi <qwjhust@gmail.com> wrote:
>
> Yes.  The current path removes the folio from the page cache, so a later
> mmap access can incur both another fault and I/O.  I should have stated
> that explicitly.
>
> For fault-around neighbors, folio_mapped() cannot distinguish the faulting
> page from a speculative neighbor.  This patch takes the conservative policy
> that an installed PTE represents a competing cache user and wins over the
> writer's RWF_DONTCACHE hint.  It can therefore retain a speculative neighbor
> which is never accessed.  Distinguishing those cases seems to require
> fault-around to preserve the faulting folio while not mapping, or later
> dropping, dropbehind neighbors.  I think that should be considered as a
> separate follow-up.

Yes. Maybe we can skip mapping neighbors with the dropbehind flag in
fault-around.

>
> For folio_launder(), filemap_end_dropbehind() holds the folio lock and returns
> if the folio is dirty or under writeback before calling
> folio_unmap_invalidate().  folio_launder() also immediately returns for a
> clean folio.  I do not see how nfs_launder_folio() is reached from this
> completion path unless the locked, unmapped folio can become dirty between
> those checks.  Is there a path I am missing?

You are probably right. I was referring to the bit wait in
nfs_launder_folio(), though I'm not quite sure whether this can
actually happen:
folio_wait_private_2(folio); /* [DEPRECATED] */

I assume we won't wait for `private_2` while the folio is clean. If so,
it should be fine.

Re: [PATCH v4] mm: filemap: retain mapped dropbehind folios

[Tal Zussman]

On 8/30/26 6:25 AM, Wenjie Qi wrote:
> From: Wenjie Qi <qiwenjie@ xiaomi. com> Fault-around can map ready dropbehind 
> folios without going through the normal page-cache lookup that clears 
> dropbehind. A mapping represents a competing cached user, but writeback 
> completion can currently
> ZjQcmQRYFpfptBannerStart
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> 
> From: Wenjie Qi <qiwenjie@xiaomi.com>
> 
> Fault-around can map ready dropbehind folios without going through the
> normal page-cache lookup that clears dropbehind. A mapping represents a
> competing cached user, but writeback completion can currently unmap that
> folio. A later mmap access must then fault it back in.
> 
> Retain mapped folios instead. For a mapped folio,
> folio_unmap_invalidate() can call unmap_mapping_folio(), which takes
> i_mmap_rwsem and may sleep. Retaining the folio also avoids this path when
> folio_end_dropbehind() runs in non-preemptible task context.
> 
> Unmapped dropbehind folios continue through the existing invalidation path.
> 
> Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Tal Zussman <tz2294@columbia.edu>
> ---
> The mapped-plus-dropbehind state was reproduced in QEMU. The patched
> kernel retained the mapped folio and continued to evict the unmapped
> dontcache folio. I did not reproduce the sleeping-in-atomic warning and
> am not aware of an existing report.
> 

I was able to trigger the sleeping-while-atomic BUG on current mainline with
some help from Claude. The reproducer itself is straightforward enough, but the
configuration was a little finicky. The atomic context comes from using
threadirqs so that virtblk_done() completes requests from the irq thread under
local_bh_disable() and the vq spinlock with irqs off, so in_task() is true but
the i_mmap_rwsem down_read() sleeps. It also needed 1 vCPU (so completion stays
in the irq thread rather than softirq) and ext4 -o dioread_lock (so end_io runs
inline instead of on a workqueue). With v4 applied the bug is gone, so:

Tested-by: Tal Zussman <tz2294@columbia.edu>

Full splat:

[   46.745106] BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:1573
[   46.745183] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 61, name: irq/40-virtio1-
[   46.745211] preempt_count: 201, expected: 0
[   46.745233] RCU nest depth: 0, expected: 0
[   46.745292] locks held by irq/40-virtio1-/61: 1, last CPU#0:
[   46.745324]  #0: ff1fce9901fc2f20 (&vblk->vqs[i].lock){....}-{3:3}, at: virtblk_done+0x56/0x100
[   46.746801] irq event stamp: 1585
[   46.748015] hardirqs last  enabled at (1583): [<ffffffff88dc7c54>] finish_task_switch.isra.0+0xd4/0x360
[   46.748224] hardirqs last disabled at (1585): [<ffffffff88e2b16c>] irq_forced_thread_fn+0x5c/0x60
[   46.748405] softirqs last  enabled at (1578): [<ffffffff88e2b159>] irq_forced_thread_fn+0x49/0x60
[   46.748580] softirqs last disabled at (1584): [<ffffffff88e2b12d>] irq_forced_thread_fn+0x1d/0x60
[   46.749233] CPU: 0 UID: 0 PID: 61 Comm: irq/40-virtio1- Not tainted 7.2.0+ #5 PREEMPT(full)
[   46.749323] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   46.749472] Call Trace:
[   46.749552]  <TASK>
[   46.749653]  dump_stack_lvl+0x66/0xa0
[   46.749747]  __might_resched+0x154/0x260
[   46.749787]  down_read+0x1e/0x170
[   46.749806]  unmap_mapping_folio+0x74/0xd0
[   46.749854]  folio_unmap_invalidate+0x92/0x200
[   46.749886]  folio_end_writeback+0x51/0xb0
[   46.749906]  ext4_finish_bio+0x36d/0x3a0
[   46.749979]  ext4_end_bio+0x51/0x140
[   46.750010]  blk_update_request+0x101/0x4d0
[   46.750045]  ? detach_buf_split_in_order+0x18c/0x1b0
[   46.750098]  blk_mq_end_request+0x20/0x130
[   46.750135]  virtblk_done+0x75/0x100
[   46.750173]  ? irq_thread+0xb3/0x2a0
[   46.750190]  vring_interrupt+0x8c/0xd0
[   46.750205]  irq_thread_fn+0x23/0x60
[   46.750238]  irq_forced_thread_fn+0x35/0x60
[   46.750261]  irq_thread+0x17c/0x2a0
[   46.750274]  ? __pfx_irq_forced_thread_fn+0x10/0x10
[   46.750299]  ? __pfx_irq_thread_dtor+0x10/0x10
[   46.750333]  ? __pfx_irq_thread+0x10/0x10
[   46.750353]  kthread+0xf6/0x130
[   46.750369]  ? __pfx_kthread+0x10/0x10
[   46.750404]  ret_from_fork+0x248/0x350
[   46.750421]  ? __pfx_kthread+0x10/0x10
[   46.750442]  ret_from_fork_asm+0x1a/0x30
[   46.750585]  </TASK>

[   46.752964] =============================
[   46.753047] [ BUG: Invalid wait context ]
[   46.753230] 7.2.0+ #5 Tainted: G        W
[   46.753353] -----------------------------
[   46.753438] irq/40-virtio1-/61 is trying to lock:
[   46.753533] ff1fce99004e4990 (&mapping->i_mmap_rwsem){++++}-{4:4}, at: unmap_mapping_folio+0x74/0xd0
[   46.753882] other info that might help us debug this:
[   46.754000] context-{5:5}
[   46.754062] locks held by irq/40-virtio1-/61: 1, last CPU#0:
[   46.754171]  #0: ff1fce9901fc2f20 (&vblk->vqs[i].lock){....}-{3:3}, at: virtblk_done+0x56/0x100
[   46.754371] stack backtrace:
[   46.754441] CPU: 0 UID: 0 PID: 61 Comm: irq/40-virtio1- Tainted: G        W           7.2.0+ #5 PREEMPT(full)
[   46.754656] Tainted: [W]=WARN
[   46.754725] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   46.754887] Call Trace:
[   46.754942]  <TASK>
[   46.754996]  dump_stack_lvl+0x66/0xa0
[   46.755082]  __lock_acquire+0x92e/0x1a80
[   46.755197]  lock_acquire+0xcd/0x2d0
[   46.755282]  ? unmap_mapping_folio+0x74/0xd0
[   46.755403]  down_read+0x42/0x170
[   46.755476]  ? unmap_mapping_folio+0x74/0xd0
[   46.755565]  unmap_mapping_folio+0x74/0xd0
[   46.755664]  folio_unmap_invalidate+0x92/0x200
[   46.755763]  folio_end_writeback+0x51/0xb0
[   46.755851]  ext4_finish_bio+0x36d/0x3a0
[   46.755958]  ext4_end_bio+0x51/0x140
[   46.756043]  blk_update_request+0x101/0x4d0
[   46.756135]  ? detach_buf_split_in_order+0x18c/0x1b0
[   46.756254]  blk_mq_end_request+0x20/0x130
[   46.756349]  virtblk_done+0x75/0x100
[   46.756438]  ? irq_thread+0xb3/0x2a0
[   46.756515]  vring_interrupt+0x8c/0xd0
[   46.756595]  irq_thread_fn+0x23/0x60
[   46.756678]  irq_forced_thread_fn+0x35/0x60
[   46.756768]  irq_thread+0x17c/0x2a0
[   46.756842]  ? __pfx_irq_forced_thread_fn+0x10/0x10
[   46.756945]  ? __pfx_irq_thread_dtor+0x10/0x10
[   46.757045]  ? __pfx_irq_thread+0x10/0x10
[   46.757136]  kthread+0xf6/0x130
[   46.757207]  ? __pfx_kthread+0x10/0x10
[   46.757302]  ret_from_fork+0x248/0x350
[   46.757383]  ? __pfx_kthread+0x10/0x10
[   46.757466]  ret_from_fork_asm+0x1a/0x30
[   46.757594]  </TASK>


> Changes since v3:
> - Describe the forced-unmap/refault runtime effect.
> - Record the reproduction boundary and add Cc: stable@vger.kernel.org.
> 
>   mm/filemap.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6afec636881f..00fd89cf6f55 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struct folio *folio)
>   		return;
>   	if (!folio_test_clear_dropbehind(folio))
>   		return;
> -	if (mapping)
> +	if (mapping && !folio_mapped(folio))
>   		folio_unmap_invalidate(mapping, folio, 0);
>   }
>   
> -- 
> 2.43.0
>