On Fri 24-06-22 14:51:18, Christoph Hellwig wrote: > On Fri, Jun 24, 2022 at 08:30:00PM +0800, Qu Wenruo wrote: > > But from my previous feedback on subpage code, it looks like it's some > > hardware archs (S390?) that can not do page flags update atomically. > > > > I have tested similar thing, with extra ASSERT() to make sure the cow > > fixup code never get triggered. > > > > At least for x86_64 and aarch64 it's OK here. > > > > So I hope this time we can get a concrete reason on why we need the > > extra page Private2 bit in the first place. > > I don't think atomic page flags are a thing here. I remember Jan > had chased a bug where we'd get into trouble into this area in > ext4 due to the way pages are locked down for direct I/O, but I > don't even remember seeing that on XFS. Either way the PageOrdered > check prevents a crash in that case and we really can't expect > data to properly be written back in that case. I'm not sure I get the context 100% right but pages getting randomly dirty behind filesystem's back can still happen - most commonly with RDMA and similar stuff which calls set_page_dirty() on pages it has got from pin_user_pages() once the transfer is done. page_maybe_dma_pinned() should be usable within filesystems to detect such cases and protect the filesystem but so far neither me nor John Hubbart has got to implement this in the generic writeback infrastructure + some filesystem as a sample case others could copy... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
