Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio

[Barry Song]

On Tue, Mar 5, 2024 at 10:02 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 04.03.24 21:42, Barry Song wrote:
> > On Tue, Mar 5, 2024 at 3:27 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 04.03.24 14:03, Ryan Roberts wrote:
> >>> On 04/03/2024 12:41, David Hildenbrand wrote:
> >>>> On 04.03.24 13:20, Ryan Roberts wrote:
> >>>>> Hi Barry,
> >>>>>
> >>>>> On 04/03/2024 10:37, Barry Song wrote:
> >>>>>> From: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>>
> >>>>>> page_vma_mapped_walk() within try_to_unmap_one() races with other
> >>>>>> PTEs modification such as break-before-make, while iterating PTEs
> >>>>>> of a large folio, it will only begin to acquire PTL after it gets
> >>>>>> a valid(present) PTE. break-before-make intermediately sets PTEs
> >>>>>> to pte_none. Thus, a large folio's PTEs might be partially skipped
> >>>>>> in try_to_unmap_one().
> >>>>>
> >>>>> I just want to check my understanding here - I think the problem occurs for
> >>>>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now
> >>>>> that I've had a look at the code and have a better understanding, I think that
> >>>>> must be the case? And therefore this problem exists independently of my work to
> >>>>> support swap-out of mTHP? (From your previous report I was under the impression
> >>>>> that it only affected mTHP).
> >>>>>
> >>>>> Its just that the problem is becoming more pronounced because with mTHP,
> >>>>> PTE-mapped large folios are much more common?
> >>>>
> >>>> That is my understanding.
> >>>>
> >>>>>
> >>>>>> For example, for an anon folio, after try_to_unmap_one(), we may
> >>>>>> have PTE0 present, while PTE1 ~ PTE(nr_pages - 1) are swap entries.
> >>>>>> So folio will be still mapped, the folio fails to be reclaimed.
> >>>>>> What’s even more worrying is, its PTEs are no longer in a unified
> >>>>>> state. This might lead to accident folio_split() afterwards. And
> >>>>>> since a part of PTEs are now swap entries, accessing them will
> >>>>>> incur page fault - do_swap_page.
> >>>>>> It creates both anxiety and more expense. While we can't avoid
> >>>>>> userspace's unmap to break up unified PTEs such as CONT-PTE for
> >>>>>> a large folio, we can indeed keep away from kernel's breaking up
> >>>>>> them due to its code design.
> >>>>>> This patch is holding PTL from PTE0, thus, the folio will either
> >>>>>> be entirely reclaimed or entirely kept. On the other hand, this
> >>>>>> approach doesn't increase PTL contention. Even w/o the patch,
> >>>>>> page_vma_mapped_walk() will always get PTL after it sometimes
> >>>>>> skips one or two PTEs because intermediate break-before-makes
> >>>>>> are short, according to test. Of course, even w/o this patch,
> >>>>>> the vast majority of try_to_unmap_one still can get PTL from
> >>>>>> PTE0. This patch makes the number 100%.
> >>>>>> The other option is that we can give up in try_to_unmap_one
> >>>>>> once we find PTE0 is not the first entry we get PTL, we call
> >>>>>> page_vma_mapped_walk_done() to end the iteration at this case.
> >>>>>> This will keep the unified PTEs while the folio isn't reclaimed.
> >>>>>> The result is quite similar with small folios with one PTE -
> >>>>>> either entirely reclaimed or entirely kept.
> >>>>>> Reclaiming large folios by holding PTL from PTE0 seems a better
> >>>>>> option comparing to giving up after detecting PTL begins from
> >>>>>> non-PTE0.
> >>>>>>
> >>>>
> >>>> I'm sure that wall of text can be formatted in a better way :) . Also, I think
> >>>> we can drop some of the details,
> >>>>
> >>>> If you need some inspiration, I can give it a shot.
> >>>>
> >>>>>> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> >>>>>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> >>>>>
> >>>>> Do we need a Fixes tag?
> >>>>>
> >>>>
> >>>> What would be the description of the problem we are fixing?
> >>>>
> >>>> 1) failing to unmap?
> >>>>
> >>>> That can happen with small folios as well IIUC.
> >>>>
> >>>> 2) Putting the large folio on the deferred split queue?
> >>>>
> >>>> That sounds more reasonable.
> >>>
> >>> Isn't the real problem today that we can end up writng a THP to the swap file
> >>> (so 2M more IO and space used) but we can't remove it from memory, so no actual
> >>> reclaim happens? Although I guess your (2) is really just another way of saying
> >>> that.
> >>
> >> The same could happen with small folios I believe? We might end up
> >> running into the
> >>
> >> folio_mapped()
> >>
> >> after the try_to_unmap().
> >>
> >> Note that the actual I/O does not happen during add_to_swap(), but
> >> during the pageout() call when we find the folio to be dirty.
> >>
> >> So there would not actually be more I/O. Only swap space would be
> >> reserved, that would be used later when not running into the race.
> >
> > I am not worried about small folios at all as they have only one PTE.
> > so the PTE is either completely unmapped or completely mapped.
> >
> > In terms of large folios, it is a different story. for example, a large
> > folio with 16 PTEs with CONT-PTE, we will have
> >
> > 1. unfolded CONT-PTE, eg. PTE0 present, PTE1-PTE15 swap entries
> >
> > 2. page faults on PTE1-PTE15 after try_to_unmap if we access them.
> >
> > This is totally useless PF and can be avoided if we can try_to_unmap
> > properly at the beginning.
> >
> > 3. potential need to split a large folio afterwards. for example, MADV_PAGEOUT,
> > MADV_FREE might split it after finding it is not completely mapped.
> >
> > For small folios, we don't have any concern on the above issues.
>
> Right, but when we talk about "Fixes:", what exactly are we consider
> "really broken" above and what is "undesired"?
>
> (a) is there a correctness issue? I don't think so.
>
> (b) is there a real performance issue? I'd like to understand.
>
> After all, we've been living with that ever since we supported THP_SWAP,
> correct? "something does not work ideally in some corner cases" might be
> reasonable to handle here (and I really think we should), but might not
> be worth a "Fixes:".
>
> So if we could clarify that, it would be great.

I don't think this needs a fixes tag, and I would think it is an optimization
on corner cases. And I don't think we will see noticeable performance
improvement as this isn't happening quite often though I believe it does
improve the performance of corner cases. but if the corner case only
takes 0.1% of all try_to_unmap_one, no noticeable performance
improvement will be seen.

I feel it is more of a behavior to kick flies out while flies don't kill people
but it can be sometimes quite annoying :-)  I ran into another bug in
large-folio swapin series due to this problem, 16 PTEs had contiguous
swap entries from Ryan's add_to_swap(), but they were splitted in
MADV_PAGEOUT because the folio was not completely mapped after
try_to_unmap_one.

Then after some time, some of the 16 pages were in swapcache, while
others were not in. In this case, I couldn't swap them in together and had to
handle PF one by one, but i was incorrectly handling it  and trying to
swap-in them together by reading swapfile. if they were atomically
handled in try_to_unmap_one, we could avoid this.

we may have to cope with this kind of problem from time to time in
future work.

>
> --
> Cheers,
>
> David / dhildenb

Thanks
Barry