Re: [RFC PATCH] mm/shmem: Fix undo range for failed fallocate

[hev <r@hev.cc>]

Hi Matthew,

On Wed, Nov 2, 2022 at 10:41 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Nov 01, 2022 at 11:22:48AM +0800, Rui Wang wrote:
> > This patch fixes data loss caused by the fallocate system
> > call interrupted by a signal.
> >
> > Bug: https://lore.kernel.org/linux-mm/33b85d82.7764.1842e9ab207.Coremail.chenguoqic@163.com/
> > Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")
>
> How does that commit introduce this bug?

In the test case[1], we created a file that contains non-zero data
from offset 0 to A-1. and a process try to expand this file by
fallocate(fd, 0, 0, B), B > A.
Concurrently, another process try to interrupt this fallocate syscall
by a signal. I think the expected results are:

1. The file is not expanded and file size is A, and the data from
offset 0 to A-1 is not changed.
2. The file is expanded and the data from offset 0 to A-1 is not
changed, and from A to B-1 contains zeros.

Now, the unexpected result is that the file is not expanded and the
data that from offset 0 to A-1 is changed by
truncate_inode_partial_folio that called
from shmem_undo_range with unfalloc = true.

This issue is only reproduced when file on tmpfs, and begin from this
commit: b9a8a4195c7d ("truncate,shmem: Handle truncates that split
large folios")

>
> > Reported-by: Guoqi Chen <chenguoqic@163.com>
> > Cc: Huacai Chen <chenhuacai@loongson.cn>
> > Signed-off-by: Rui Wang <kernel@hev.cc>
> > ---
> >  mm/shmem.c | 20 ++++++++++++--------
> >  1 file changed, 12 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index bc9b84602eec..8c8dce34eafc 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -948,11 +948,13 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
> >       folio = shmem_get_partial_folio(inode, lstart >> PAGE_SHIFT);
> >       if (folio) {
> >               same_folio = lend < folio_pos(folio) + folio_size(folio);
> > -             folio_mark_dirty(folio);
> > -             if (!truncate_inode_partial_folio(folio, lstart, lend)) {
> > -                     start = folio->index + folio_nr_pages(folio);
> > -                     if (same_folio)
> > -                             end = folio->index;
> > +             if (!unfalloc || !folio_test_uptodate(folio)) {
> > +                     folio_mark_dirty(folio);
> > +                     if (!truncate_inode_partial_folio(folio, lstart, lend)) {
> > +                             start = folio->index + folio_nr_pages(folio);
> > +                             if (same_folio)
> > +                                     end = folio->index;
> > +                     }
>
> ... so what you're saying is that if we allocate a page, but zeroing
> it is interrupted by a signal, we cannot now remove that page from
> the cache?  That seems wrong.
>
> Surely the right solution is to remove this page from the cache if we're
> interrupted by a signal.

So I think we should not truncate_inode_partial_folio for unfalloc =
true. Isn't that right?

[1] https://github.com/abner-chenc/abner/blob/master/fallocate.c

Regards,
Ray