Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Thu, May 18, 2023 at 06:23:44AM -0700, Christoph Hellwig wrote: >> On Wed, May 17, 2023 at 02:48:12PM -0400, Brian Foster wrote: >> > But I also wonder.. if we can skip the iop alloc on full folio buffered >> > overwrites, isn't that also true of mapped writes to folios that don't >> > already have an iop? >> >> Yes. > > Hm, well, maybe? If somebody stores to a page, we obviously set the > dirty flag on the folio, but depending on the architecture, we may > or may not have independent dirty bits on the PTEs (eg if it's a PMD, > we have one dirty bit for the entire folio; similarly if ARM uses the > contiguous PTE bit). If we do have independent dirty bits, we could > dirty only the blocks corresponding to a single page at a time. > > This has potential for causing some nasty bugs, so I'm inclined to > rule that if a folio is mmaped, then it's all dirty from any writable > page fault. The fact is that applications generally do not perform > writes through mmap because the error handling story is so poor. > > There may be a different answer for anonymous memory, but that doesn't > feel like my problem and shouldn't feel like any FS developer's problem. Although I am skeptical too to do the changes which Brian is suggesting here. i.e. not making all the blocks of the folio dirty when we are going to call ->dirty_folio -> filemap_dirty_folio() (mmaped writes). However, I am sorry but I coudn't completely follow your reasoning above. I think what Brian is suggesting here is that filemap_dirty_folio() should be similar to complete buffered overwrite case where we do not allocate the iop at the ->write_begin() time. Then at the writeback time we allocate an iop and mark all blocks dirty. In a way it is also the similar case as for mmapped writes too but my only worry is the way mmaped writes work and it makes more sense to keep the dirty state of folio and per-block within iop in sync. For that matter, we can even just make sure we always allocate an iop in the complete overwrites case as well. I didn't change that code because it was kept that way for uptodate state as well and based on one of your inputs for complete overwrite case. Though I agree that we should ideally be allocatting & marking all blocks in iop as dirty in the call to ->dirty_folio(), I just wanted to understand your reasoning better. Thanks! -ritesh
