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

[Ryan Roberts]

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?

> 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.
> 
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>

Do we need a Fixes tag?

> ---
>  mm/vmscan.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0b888a2afa58..e4722fbbcd0c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1270,6 +1270,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  
>  			if (folio_test_pmd_mappable(folio))
>  				flags |= TTU_SPLIT_HUGE_PMD;
> +			/*
> +			 * if page table lock is not held from the first PTE of
> +			 * a large folio, some PTEs might be skipped because of
> +			 * races with break-before-make, for example, PTEs can
> +			 * be pte_none intermediately, thus one or more PTEs
> +			 * might be skipped in try_to_unmap_one, we might result
> +			 * in a large folio is partially mapped and partially
> +			 * unmapped after try_to_unmap
> +			 */
> +			if (folio_test_large(folio))
> +				flags |= TTU_SYNC;

This looks sensible to me after thinking about it for a while. But I also have a
gut feeling that there might be some more subtleties that are going over my
head, since I'm not expert in this area. So will leave others to provide R-b :)

Thanks,
Ryan

>  
>  			try_to_unmap(folio, flags);
>  			if (folio_mapped(folio)) {