On Fri 24-06-22 21:19:04, Qu Wenruo wrote: > > > On 2022/6/24 21:07, Jan Kara wrote: > > 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. > > Just curious, things like RMDA can mark those pages dirty even without > letting kernel know, but how could those pages be from page cache? By > mmap()? Yes, you pass virtual address to RDMA ioctl and it uses memory at that address as a target buffer for RDMA. If the target address happens to be mmapped file, filesystem has problems... > > 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... > > So the generic idea is just to detect if the page is marked dirty by > traditional means, and if not, skip the writeback for them, and wait for > proper notification to fs? Kind of. The idea is to treat page_maybe_dma_pinned() pages as permanently dirty (because we have no control over when the hardware decides to modify the page contents by DMA). So skip the writeback if we can (e.g. memory cleaning type of writeback) and use bounce pages to do data integrity writeback (which cannot be skipped). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
