Re: [PATCH v2 2/2] iomap: make zero range flush conditional on unwritten mappings

Brian Foster
2024-08-30T11:57:53Z

On Thu, Aug 29, 2024 at 02:34:02PM -0700, Darrick J. Wong wrote:
> On Thu, Aug 29, 2024 at 11:03:57AM -0400, Brian Foster wrote:
> > On Wed, Aug 28, 2024 at 03:44:20PM -0700, Darrick J. Wong wrote:
> > > On Wed, Aug 28, 2024 at 02:19:11PM -0400, Brian Foster wrote:
> > > > iomap_zero_range() flushes pagecache to mitigate consistency
> > > > problems with dirty pagecache and unwritten mappings. The flush is
> > > > unconditional over the entire range because checking pagecache state
> > > > after mapping lookup is racy with writeback and reclaim. There are
> > > > ways around this using iomap's mapping revalidation mechanism, but
> > > > this is not supported by all iomap based filesystems and so is not a
> > > > generic solution.
> > > >
> > > > There is another way around this limitation that is good enough to
> > > > filter the flush for most cases in practice. If we check for dirty
> > > > pagecache over the target range (instead of unconditionally flush),
> > > > we can keep track of whether the range was dirty before lookup and
> > > > defer the flush until/unless we see a combination of dirty cache
> > > > backed by an unwritten mapping. We don't necessarily know whether
> > > > the dirty cache was backed by the unwritten maping or some other
> > > > (written) part of the range, but the impliciation of a false
> > > > positive here is a spurious flush and thus relatively harmless.
> > > >
> > > > Note that we also flush for hole mappings because iomap_zero_range()
> > > > is used for partial folio zeroing in some cases. For example, if a
> > > > folio straddles EOF on a sub-page FSB size fs, the post-eof portion
> > > > is hole-backed and dirtied/written via mapped write, and then i_size
> > > > increases before writeback can occur (which otherwise zeroes the
> > > > post-eof portion of the EOF folio), then the folio becomes
> > > > inconsistent with disk until reclaimed. A flush in this case
> > > > executes partial zeroing from writeback, and iomap knows that there
> > > > is otherwise no I/O to submit for hole backed mappings.
> > > >
> > > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > > ---
> > > > fs/iomap/buffered-io.c | 57 +++++++++++++++++++++++++++++++++++-------
> > > > 1 file changed, 48 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > > index 3e846f43ff48..a6e897e6e303 100644
> > > > --- a/fs/iomap/buffered-io.c
> > > > +++ b/fs/iomap/buffered-io.c
> > > > @@ -1393,16 +1393,47 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
> > > > }
> > > > EXPORT_SYMBOL_GPL(iomap_file_unshare);
> > > >
> > > > -static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
> > > > +/*
> > > > + * Flush the remaining range of the iter and mark the current mapping stale.
> > > > + * This is used when zero range sees an unwritten mapping that may have had
> > > > + * dirty pagecache over it.
> > > > + */
> > > > +static inline int iomap_zero_iter_flush_and_stale(struct iomap_iter *i)
> > > > +{
> > > > + struct address_space *mapping = i->inode->i_mapping;
> > > > + loff_t end = i->pos + i->len - 1;
> > > > +
> > > > + i->iomap.flags |= IOMAP_F_STALE;
> > > > + return filemap_write_and_wait_range(mapping, i->pos, end);
> > > > +}
> > > > +
> > > > +static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
> > > > + bool *range_dirty)
> > > > {
> > > > const struct iomap *srcmap = iomap_iter_srcmap(iter);
> > > > loff_t pos = iter->pos;
> > > > loff_t length = iomap_length(iter);
> > > > loff_t written = 0;
> > > >
> > > > - /* already zeroed? we're done. */
> > > > - if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> > > > + /*
> > > > + * We can skip pre-zeroed mappings so long as either the mapping was
> > > > + * clean before we started or we've flushed at least once since.
> > > > + * Otherwise we don't know whether the current mapping had dirty
> > > > + * pagecache, so flush it now, stale the current mapping, and proceed
> > > > + * from there.
> > > > + *
> > > > + * The hole case is intentionally included because this is (ab)used to
> > > > + * handle partial folio zeroing in some cases. Hole backed post-eof
> > > > + * ranges can be dirtied via mapped write and the flush triggers
> > > > + * writeback time post-eof zeroing.
> > > > + */
> > > > + if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN) {
> > > > + if (*range_dirty) {
> > > > + *range_dirty = false;
> > > > + return iomap_zero_iter_flush_and_stale(iter);
> > > > + }
> > > > return length;
> > > > + }
> > > >
> > > > do {
> > > > struct folio *folio;
> > > > @@ -1450,19 +1481,27 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> > > > .flags = IOMAP_ZERO,
> > > > };
> > > > int ret;
> > > > + bool range_dirty;
> > > >
> > > > /*
> > > > * Zero range wants to skip pre-zeroed (i.e. unwritten) mappings, but
> > > > * pagecache must be flushed to ensure stale data from previous
> > > > - * buffered writes is not exposed.
> > > > + * buffered writes is not exposed. A flush is only required for certain
> > > > + * types of mappings, but checking pagecache after mapping lookup is
> > > > + * racy with writeback and reclaim.
> > > > + *
> > > > + * Therefore, check the entire range first and pass along whether any
> > > > + * part of it is dirty. If so and an underlying mapping warrants it,
> > > > + * flush the cache at that point. This trades off the occasional false
> > > > + * positive (and spurious flush, if the dirty data and mapping don't
> > > > + * happen to overlap) for simplicity in handling a relatively uncommon
> > > > + * situation.
> > > > */
> > > > - ret = filemap_write_and_wait_range(inode->i_mapping,
> > > > - pos, pos + len - 1);
> > > > - if (ret)
> > > > - return ret;
> > > > + range_dirty = filemap_range_needs_writeback(inode->i_mapping,
> > > > + pos, pos + len - 1);
> > > >
> > > > while ((ret = iomap_iter(&iter, ops)) > 0)
> > > > - iter.processed = iomap_zero_iter(&iter, did_zero);
> > > > + iter.processed = iomap_zero_iter(&iter, did_zero, &range_dirty);
> > >
> > > Style nit: Could we do this flush-and-stale from the loop body instead
> > > of passing pointers around? e.g.
> > >
> >
> > So FWIW, I had multiple other variations of this that used an
> > IOMAP_DIRTY_CACHE flag on the iomap to track dirty pagecache for
> > arbitrary operations. The flag could be set and cleared at the
> > appropriate points as expected (for ops that care).
> >
> > To me, that's how I'd prefer to avoid just passing a pointer, but I
> > intentionally factored that out to avoid using a flag for something that
> > (for now) could be simplified to a local variable. OTOH, it is something
> > that might be useful for the iomap seek data/hole implementations down
> > the road.
>
> <nod> We can always adjust again when we get there; for now a local
> variable sounds fine.
>
> > I've played with that a bit, but also have been trying to avoid getting
> > too much into that rabbit hole for zero range. My thought was I'd
> > reintroduce it and replace the range_dirty thing if/when it proved
> > useful for multiple operations.
> >
> > > static inline bool iomap_zero_need_flush(const struct iomap_iter *i)
> > > {
> > > const struct iomap *srcmap = iomap_iter_srcmap(iter);
> > >
> > > return srcmap->type == IOMAP_HOLE ||
> > > srcmap->type == IOMAP_UNWRITTEN;
> > > }
> >
> > The factoring looks mostly reasonable, but a couple things bug me that
> > I'd like to see if we can resolve..
> >
> > One is that this doesn't really indicate whether a flush is needed,
> > because the dirty cache state is a critical part of that logic. I
> > suppose we could rename it (to what?), but it also seems a little odd to
> > have a helper just for mapping type checks.
>
> I thought about passing range_dirty into iomap_zero_need_flush since
> it's a static inline function, but that just seemed unnecessary.
>
> > > static inline int iomap_zero_iter_flush(struct iomap_iter *i)
> > > {
> > > struct address_space *mapping = i->inode->i_mapping;
> > > loff_t end = i->pos + i->len - 1;
> > >
> > > i->iomap.flags |= IOMAP_F_STALE;
> > > return filemap_write_and_wait_range(mapping, i->pos, end);
> > > }
> > >
> > > and then:
> > >
> > > range_dirty = filemap_range_needs_writeback(...);
> > >
> > > while ((ret = iomap_iter(&iter, ops)) > 0) {
> > > if (range_dirty && iomap_zero_need_flush(&iter)) {
> > > /*
> > > * Zero range wants to skip pre-zeroed (i.e.
> > > * unwritten) mappings, but...
> > > */
> > > range_dirty = false;
> > > iter.processed = iomap_zero_iter_flush(&iter);
> > > } else {
> > > iter.processed = iomap_zero_iter(&iter, did_zero);
> > > }
> >
> > The other is that the optimization logic is now split across multiple
> > functions. I.e., iomap_zero_iter() has a landmine if ever called without
> > doing the flush_and_stale() part first (a consideration if
> > truncate_page() were ever open coded, for example).
>
> _zero_iter is a static function, let's hope nobody does that. Though
> you're right, experience tells me that someone will try this
> eventually.
>

Yeah, it's probably unlikely, but the fact I already had the open coded
iomap_truncate_page() experiment (from the v1 thread) lying around that
does pretty much this is what gave me pause.

> That said, I see the merit of having one complete loop body function
> that knows how to handle all iomap types, since the others do that.
>
> > I wonder if a compromise might be to factor out the whole optimization
> > into a separate helper rather than just the flush part (first via a prep
> > patch), then the higher level loop ends up looking almost the same:
> >
> > while ((ret = iomap_iter(&iter, ops)) > 0) {
> > /* special handling for already zeroed mappings */
> > if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
> > iter.processed = iomap_zero_mapping_iter(&iter, &range_dirty);
> > else
> > iter.processed = iomap_zero_iter(&iter, did_zero);
> > }
> >
> > That doesn't avoid passing the range_dirty pointer, but we just end up
> > passing that instead of did_zero. Also as noted above, it could still be
> > made to go away if the range_dirty check gets pushed down into the
> > iomap_iter() path for more general use.
> >
> > Anyways those are just my thoughts. I'm of the mind that whatever
> > factoring we do here may have to change if Dave's batched folio
> > lookup/iteration idea pans out for fs' with validation support, so at
> > the end of the day I'll change this to look exactly like you wrote it if
> > it means the zeroing problem gets fixed. Thoughts or preference?
>
> I'm ok with your original version now.
>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
>

Thanks. I'll post a v3 just with Dave's comment updates then.

Brian

> --D
>
>
> > Brian
> >
> > > }
> > >
> > > The logic looks correct and sensible. :)
> > >
> > > --D
> > >
> > > > return ret;
> > > > }
> > > > EXPORT_SYMBOL_GPL(iomap_zero_range);
> > > > --
> > > > 2.45.0
> > > >
> > > >
> > >
> >
> >
>