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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

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?

>
> 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
>> > >
>> >
>>