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

[Matthew Wilcox]

On Sat, Nov 19, 2022 at 10:45:52PM +0800, hev wrote:
> Ping

I'll be back on Wednesday and will look then.

> On Thu, Nov 3, 2022 at 3:52 PM hev <r@xxxxxx> wrote:
> >
> > Hi Matthew,
> >
> > On Wed, Nov 2, 2022 at 10:41 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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@xxxxxxx/
> > > > 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@xxxxxxx>
> > > > Cc: Huacai Chen <chenhuacai@xxxxxxxxxxx>
> > > > Signed-off-by: Rui Wang <kernel@xxxxxx>
> > > > ---
> > > >  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
>