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
