[patch 123/262] mm: filemap: coding style cleanup for filemap_map_pmd()

Andrew Morton
2021-11-05T20:41:08Z

From: Yang Shi <shy828301@gmail.com>
Subject: mm: filemap: coding style cleanup for filemap_map_pmd()

Patch series "Solve silent data loss caused by poisoned page cache (shmem/tmpfs)", v5.

When discussing the patch that splits page cache THP in order to offline
the poisoned page, Noaya mentioned there is a bigger problem [1] that
prevents this from working since the page cache page will be truncated if
uncorrectable errors happen. By looking this deeper it turns out this
approach (truncating poisoned page) may incur silent data loss for all
non-readonly filesystems if the page is dirty. It may be worse for
in-memory filesystem, e.g. shmem/tmpfs since the data blocks are actually
gone.

To solve this problem we could keep the poisoned dirty page in page cache
then notify the users on any later access, e.g. page fault, read/write,
etc. The clean page could be truncated as is since they can be reread
from disk later on.

The consequence is the filesystems may find poisoned page and manipulate
it as healthy page since all the filesystems actually don't check if the
page is poisoned or not in all the relevant paths except page fault. In
general, we need make the filesystems be aware of poisoned page before we
could keep the poisoned page in page cache in order to solve the data loss
problem.

To make filesystems be aware of poisoned page we should consider:
- The page should be not written back: clearing dirty flag could prevent from
writeback.
- The page should not be dropped (it shows as a clean page) by drop caches or
other callers: the refcount pin from hwpoison could prevent from invalidating
(called by cache drop, inode cache shrinking, etc), but it doesn't avoid
invalidation in DIO path.
- The page should be able to get truncated/hole punched/unlinked: it works as it
is.
- Notify users when the page is accessed, e.g. read/write, page fault and other
paths (compression, encryption, etc).

The scope of the last one is huge since almost all filesystems need do it
once a page is returned from page cache lookup. There are a couple of
options to do it:

1. Check hwpoison flag for every path, the most straightforward way.
2. Return NULL for poisoned page from page cache lookup, the most callsites
check if NULL is returned, this should have least work I think. But the
error handling in filesystems just return -ENOMEM, the error code will incur
confusion to the users obviously.
3. To improve #2, we could return error pointer, e.g. ERR_PTR(-EIO), but this
will involve significant amount of code change as well since all the paths
need check if the pointer is ERR or not just like option #1.

I did prototype for both #1 and #3, but it seems #3 may require more
changes than #1. For #3 ERR_PTR will be returned so all the callers need
to check the return value otherwise invalid pointer may be dereferenced,
but not all callers really care about the content of the page, for
example, partial truncate which just sets the truncated range in one page
to 0. So for such paths it needs additional modification if ERR_PTR is
returned. And if the callers have their own way to handle the problematic
pages we need to add a new FGP flag to tell FGP functions to return the
pointer to the page.

It may happen very rarely, but once it happens the consequence (data
corruption) could be very bad and it is very hard to debug. It seems this
problem had been slightly discussed before, but seems no action was taken
at that time. [2]

As the aforementioned investigation, it needs huge amount of work to solve
the potential data loss for all filesystems. But it is much easier for
in-memory filesystems and such filesystems actually suffer more than
others since even the data blocks are gone due to truncating. So this
patchset starts from shmem/tmpfs by taking option #1.

TODO:
* The unpoison has been broken since commit 0ed950d1f281 ("mm,hwpoison: make
get_hwpoison_page() call get_any_page()"), and this patch series make
refcount check for unpoisoning shmem page fail.
* Expand to other filesystems. But I haven't heard feedback from filesystem
developers yet.

Patch breakdown:
Patch #1: cleanup, depended by patch #2
Patch #2: fix THP with hwpoisoned subpage(s) PMD map bug
Patch #3: coding style cleanup
Patch #4: refactor and preparation.
Patch #5: keep the poisoned page in page cache and handle such case for all
the paths.
Patch #6: the previous patches unblock page cache THP split, so this patch
add page cache THP split support.

This patch (of 4):

A minor cleanup to the indent.

Link: https://lkml.kernel.org/r/20211020210755.23964-1-shy828301@gmail.com
Link: https://lkml.kernel.org/r/20211020210755.23964-4-shy828301@gmail.com
Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/filemap.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

--- a/mm/filemap.c~mm-filemap-coding-style-cleanup-for-filemap_map_pmd
+++ a/mm/filemap.c
@@ -3203,12 +3203,12 @@ static bool filemap_map_pmd(struct vm_fa
}

if (pmd_none(*vmf->pmd) && PageTransHuge(page)) {
- vm_fault_t ret = do_set_pmd(vmf, page);
- if (!ret) {
- /* The page is mapped successfully, reference consumed. */
- unlock_page(page);
- return true;
- }
+ vm_fault_t ret = do_set_pmd(vmf, page);
+ if (!ret) {
+ /* The page is mapped successfully, reference consumed. */
+ unlock_page(page);
+ return true;
+ }
}

if (pmd_none(*vmf->pmd))
_