Brian Foster <bfoster@xxxxxxxxxx> writes: > On Mon, May 22, 2023 at 05:56:25PM -0700, Darrick J. Wong wrote: >> On Mon, May 22, 2023 at 07:18:07AM -0400, Brian Foster wrote: >> > On Mon, May 22, 2023 at 10:03:05AM +0530, Ritesh Harjani wrote: >> > > 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. >> > > >> > >> > Yeah... I think what Willy is saying (i.e. to not track sub-page dirty >> > granularity of intra-folio faults) makes sense, but I'm also not sure >> > what it has to do with the idea of being consistent with how full folio >> > overwrites are implemented (between buffered or mapped writes). We're >> > not changing historical dirtying granularity either way. I think this is >> > just a bigger picture thought for future consideration as opposed to >> > direct feedback on this patch.. >> >> <nod> >> >> > > 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. >> > > >> > >> > Can you elaborate on your concerns, out of curiosity? >> > >> > Either way, IMO it also seems reasonable to drop this behavior for the >> > basic implementation of dirty tracking (so always allocate the iop for >> > sub-folio tracking as you suggest above) and then potentially restore it >> > as a separate optimization patch at the end of the series. >> >> Agree. >> >> > That said, I'm not totally clear why it exists in the first place, so >> > that might warrant some investigation. Is it primarily to defer >> > allocations out of task write/fault contexts? >> >> (Assuming by 'it' you mean the behavior where we don't unconditionally >> allocate iops for blocksize < foliosize...) >> >> IIRC the reason is to reduce memory usage by eliding iop allocations >> unless it's absolutely necessary for correctness was /my/ understanding >> of why we don't always allocate the iop... >> >> > To optimize the case where pagecache is dirtied but truncated or >> > something and thus never written back? >> >> ...because this might very well happen. Write a temporary .o file to >> the filesystem, then delete the whole thing before writeback ever gets >> its hands on the file. >> > > I don't think a simple temp write will trigger this scenario currently > because the folios would have to be uptodate at the time of the write to > bypass the iop alloc. I guess you'd have to read folios (even if backed > by holes) first to start seeing the !iop case at writeback time (for bs > != ps). > > That could change with these patches, but I was trying to reason about > the intent of the existing code and whether there was some known reason > to continue to try and defer the iop allocation as the need/complexity > for deferring it grows with the addition of more (i.e. dirty) tracking. > Here is the 1st discussion/reasoning where the deferred iop allocation in the readpage path got discussed [1]. And here is the discussion when I first pointed out the deferred allocation in writepage path. IMO, it got slipped in with the discussions maybe only on mailing list but nothing in the commit messages or comments.[2] [1]: https://lore.kernel.org/linux-xfs/20210628172727.1894503-1-agruenba@xxxxxxxxxx/ [2]: https://lore.kernel.org/linux-xfs/20230130202150.pfohy5yg6dtu64ce@rh-tp/ >> > Is there any room for further improvement where the alloc could be >> > avoided completely for folio overwrites instead of just deferred? >> >> Once writeback starts, though, we need the iop so that we can know when >> all the writeback for that folio is actually complete, no matter how >> many IOs might be in flight for that folio. I don't know how you'd get >> around this problem. >> > > Ok. I noticed some kind of counter or something being updated last time > I looked through that code, so it sounds like that's the reason the iop > eventually needs to exist. Thanks. > >> > Was that actually the case at some point and then something later >> > decided the iop was needed at writeback time, leading to current >> > behavior? >> >> It's been in iomap since the beginning when we lifted it from xfs. >> > > Not sure exactly what you're referring to here. iomap_writepage_map() > would warn on the (bs != ps && !iop) case up until commit 8e1bcef8e18d > ("iomap: Permit pages without an iop to enter writeback"), so I don't > see how iop allocs were deferred (other than when bs == ps, obviously) > prior to that. > > Heh, but I'm starting to get my wires crossed just trying to piece > things together here. Ritesh, ISTM the (writeback && !iop && bs != ps) > case is primarily a subtle side effect of the current writeback behavior > being driven by uptodate status. I think it's probably wise to drop it > at least initially, always alloc and dirty the appropriate iop ranges > for sub-folio blocks, and then if you or others think there is value in > the overwrite optimization to defer iop allocs, tack that on as a > separate patch and try to be consistent between buffered and mapped > writes. Based on the discussion so far, I would like to think of this as follow: We already have some sort of lazy iop allocation in the buffered I/O path (discussed above). This patch series does not changes that behavior. For now I would like to keep the page mkwrite page as is without any lazy iop allocation optimization. I am ok to pick this optimization work as a seperate series because, IIUC, Christoph has some ideas on deferring iop allocations even further [2] (from link shared above). Does that sound good? > > Darrick noted above that he also agrees with that separate patch > approach. For me, I think it would also be useful to show that there is > some measurable performance benefit on at least one reasonable workload > to help justify it. Agree that when we work on such optimizations as a seperate series, it will be worth measuring the performance benefits of that. -ritesh > > Brian > >> --D (who is now weeks behind on reviewing things and stressed out) >> >> > Brian >> > >> > > 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 >> > > >> > >>
