On Thu, Sep 24, 2020 at 09:12:35AM -0400, Brian Foster wrote: > On Thu, Sep 24, 2020 at 01:56:08PM +0100, Matthew Wilcox (Oracle) wrote: > > For filesystems with block size < page size, we need to set all the > > per-block uptodate bits if the page was already uptodate at the time > > we create the per-block metadata. This can happen if the page is > > invalidated (eg by a write to drop_caches) but ultimately not removed > > from the page cache. > > > > This is a data corruption issue as page writeback skips blocks which > > are marked !uptodate. > > Thanks. Based on my testing of clearing PageUptodate here I suspect this > will similarly prevent the problem, but I'll give this a test > nonetheless. > > I am a little curious why we'd prefer to fill the iop here rather than > just clear the page state if the iop data has been released. If the page > is partially uptodate, then we end up having to re-read the page > anyways, right? OTOH, I guess this behavior is more consistent with page > size == block size filesystems where iop wouldn't exist and we just go > by page state, so perhaps that makes more sense. Well, it's _true_ ... the PageUptodate bit means that every byte in this page is at least as new as every byte on storage. There's no need to re-read it, which is what we'll do if we ClearPageUptodate. My original motivation for this was splitting a THP. In that case, we have, let's say, 16 * 4kB pages, and an iop for 64 blocks. When we split that 64kB page into 16 4kB pages, we can't afford to allocate 16 iops for them, so we just drop the iop and copy the uptodate state from the head page to all subpages. So now we have 16 pages, all marked uptodate (and with valid data) but no iop. So we need to create an iop for each page during the writeback path, and that has to be created with uptodate bits or we'll skip the entire page. When I wrote the patch below, I had no idea we could already get an iop allocated for an uptodate page, or I would have submitted this patch months ago. http://git.infradead.org/users/willy/pagecache.git/commitdiff/bc503912d4a9aad4496a4591e9992f0ada47a9c9
