Re: [RFCv5 5/5] iomap: Add per-block dirty state tracking to improve performance

[Brian Foster <bfoster@redhat.com>]

On Tue, May 23, 2023 at 08:32:42PM +0530, Ritesh Harjani wrote:
> Brian Foster <bfoster@redhat.com> writes:
> 
> > On Tue, May 23, 2023 at 07:13:04PM +0530, Ritesh Harjani wrote:
> >> Brian Foster <bfoster@redhat.com> 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@infradead.org> 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@redhat.com/
> >> [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?
> >> 
> >
> > Could you do that in two steps where the buffered I/O path variant is
> > replaced by explicit dirty tracking in the initial patch, and then is
> > restored by a subsequent patch in the same series? That would allow
> 
> Sorry, I couldn't follow it. Can you please elaborate.
> 

Sorry for the confusion...

> So, what I was suggesting was - for buffered I/O path we should continue
> to have the lazy iop allocation scheme whereever possible.
> Rest of the optimizations of further deferring the iop allocation
> including in the ->mkwrite path should be dealt seperately in a later
> patch series.
> 

Yup, agree.

> Also we already have a seperate patch in this series which defers the
> iop allocation if the write completely overwrites the folio [1].
> Earlier the behavior was that it skips it entirely if the folio was
> uptodate, but since we require it for per-block dirty tracking, we
> defer iop allocation only if it was a complete overwrite of the folio. 
> 

That is a prepatory patch before iop dirty tracking is enabled in patch
5, right? I was mainly just suggesting to make the overwrite checking
part of this patch come after dirty tracking is enabled (as a small
optimization patch) rather than before.

I don't want to harp on it if that's difficult or still doesn't make
sense for some reason. I'll take a closer look the next go around when I
have a bit more time and just send a diff if it seems it can be done
cleanly..

Brian

> [1]: https://lore.kernel.org/linux-xfs/ZGzRX9YVkAYJGLqV@bfoster/T/#m048d0a097f7abb09da1c12c9a02afbcc3b9d39ee
> 
> 
> -ritesh
> 
> > keeping it around and documenting it explicitly in the commit log for
> > the separate patch, but IMO makes this a bit easier to review (and
> > potentially debug/bisect if needed down the road).
> >
> > But I don't insist if that's too troublesome for some reason...
> >
> > Brian
> >
> >> >
> >> > 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
> >> >> > >
> >> >> >
> >> >>
> >> 
>