On Wed, Feb 15, 2023 at 12:44 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Tue, Feb 14, 2023 at 04:57:09PM +0900, David Stevens wrote: > > /* > > - * At this point the hpage is locked and not up-to-date. > > - * It's safe to insert it into the page cache, because nobody would > > - * be able to map it or use it in another way until we unlock it. > > + * Mark hpage as up-to-date before inserting it into the page cache to > > + * prevent it from being mistaken for an fallocated but unwritten page. > > + * Inserting the unfinished hpage into the page cache is safe because > > + * it is locked, so nobody can map it or use it in another way until we > > + * unlock it. > > No, that's not true. The data has to be there before we mark it > uptodate. See filemap_get_pages() for example, used as part of > read(). We don't lock the page unless we need to bring it uptodate > ourselves. I've been focusing on the shmem case for collapse_file and forgot to think about the !is_shmem case. As far as I could tell, shmem doesn't use filemap_get_pages() and everything else in filemap.c/shmem.c that checks folio_test_uptodate also locks the folio. But yeah, this would break the !is_shmem case and is kind of sketchy anyway. I'll put together a better patch. -David
