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A file > cannot be open writeable and have THPs (because the filesystem is not a= ware > of them). khugepaged will never collapse file pages for files that are > opened writeable. On an open(O_RDWR/O_WRONLY), the page cache for that > particular file is dropped. This is fine because nothing could've been > dirtied. > > However, there is an edge-case: collapse_file() might not be able to > coexist with concurrent writers, but it can coexist with dirty folios > (from previous writers). Therefore, the following can happen: > > open(file, O_RDWR) > write(file) > close(file) > madvise(file_mapping, MADV_COLLAPSE, some non-dirty range) > open(file, O_RDWR) > nr_thps > 0 > truncate_inode_pages() > /* THPs are cleared out, but so are the dirty folios */ > > When this edge-case happens, there is data loss, as the dirty folios ar= e > fully discarded. > > Fix it by fully writing back the page cache (and waiting) when collapsi= ng > file THPs. Doing so provides the guarantee that no dirty folio will be > observed while there are active THPs. To fully ensure this is safe, the= > invalidate_lock needs to be held while doing the writeout, so that > do_dentry_open()'s page cache truncation excludes this write-and-wait. > > Cc: stable@vger.kernel.org > Cc: Alexander Viro > Cc: Christian Brauner > Cc: Jan Kara > Cc: Matthew Wilcox > Cc: Song Liu > Cc: Eric Hagberg > Cc: Zi Yan > Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem)= FS") > Reported-by: Gregg Leventhal > Closes: https://lore.kernel.org/linux-mm/CAFN_u7H_0ECF3jixP=3DT=3DU7AH5= =3DQ3wQNvJMo8an3VqUDMerQfUw@mail.gmail.com/ > Tested-by: Zi Yan > Signed-off-by: Pedro Falcato > --- > This patch is written against 7.1.0 (because the code no longer exists = in mainline). > > Zi, I kept your Tested-by, but I had to move some things around and > use the invalidate lock. Please re-test if you can. Tested it again on top of v6.12 (the patch applied cleanly) and the issue= is gone. My Tested-by still holds. :) > > mm/khugepaged.c | 39 +++++++++++++++++++++++++-------------- > 1 file changed, 25 insertions(+), 14 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index b8452dbdb043..0707d719a270 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2094,32 +2094,43 @@ static enum scan_result collapse_file(struct mm= _struct *mm, unsigned long addr, > goto xa_unlocked; > } > > - if (!is_shmem) { > +xa_locked: > + xas_unlock_irq(&xas); > +xa_unlocked: > + > + /* > + * If collapse is successful, flush must be done now before copying. > + * If collapse is unsuccessful, does flush actually need to be done? > + * Do it anyway, to clear the state. > + */ > + try_to_unmap_flush(); > + > + if (result =3D=3D SCAN_SUCCEED && !is_shmem) { > + /* > + * invalidate_lock as shared excludes against concurrent opens > + * in do_dentry_open() truncating the page cache. This is > + * particularly important if there are dirty folios in transit. > + */ > + filemap_invalidate_lock_shared(mapping); > filemap_nr_thps_inc(mapping); > /* > * Paired with the fence in do_dentry_open() -> get_write_access() > * to ensure i_writecount is up to date and the update to nr_thps > * is visible. Ensures the page cache will be truncated if the > - * file is opened writable. > + * file is opened writable. If collapse looks to be successful, > + * flush any dirty pages out the page cache. With the nr_thps > + * incremented, there won't be any new writers (nor new dirties). > */ > smp_mb(); > - if (inode_is_open_for_write(mapping->host)) { > + if (inode_is_open_for_write(mapping->host) || filemap_write_and_wait= (mapping)) { > result =3D SCAN_FAIL; > filemap_nr_thps_dec(mapping); > + filemap_invalidate_unlock_shared(mapping); > + goto rollback; > } > + filemap_invalidate_unlock_shared(mapping); > } > > -xa_locked: > - xas_unlock_irq(&xas); > -xa_unlocked: > - > - /* > - * If collapse is successful, flush must be done now before copying. > - * If collapse is unsuccessful, does flush actually need to be done? > - * Do it anyway, to clear the state. > - */ > - try_to_unmap_flush(); > - > if (result =3D=3D SCAN_SUCCEED && nr_none && > !shmem_charge(mapping->host, nr_none)) > result =3D SCAN_FAIL; > -- = > 2.54.0 Best Regards, Yan, Zi