Re: [PATCH 10/13] mm/munlock: mlock_page() munlock_page() batch by pagevec

[Geert Uytterhoeven]

Hi Hugh,

On Wed, Feb 9, 2022 at 3:46 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> A weakness of the page->mlock_count approach is the need for lruvec lock
> while holding page table lock.  That is not an overhead we would allow on
> normal pages, but I think acceptable just for pages in an mlocked area.
> But let's try to amortize the extra cost by gathering on per-cpu pagevec
> before acquiring the lruvec lock.
>
> I have an unverified conjecture that the mlock pagevec might work out
> well for delaying the mlock processing of new file pages until they have
> got off lru_cache_add()'s pagevec and on to LRU.
>
> The initialization of page->mlock_count is subject to races and awkward:
> 0 or !!PageMlocked or 1?  Was it wrong even in the implementation before
> this commit, which just widens the window?  I haven't gone back to think
> it through.  Maybe someone can point out a better way to initialize it.
>
> Bringing lru_cache_add_inactive_or_unevictable()'s mlock initialization
> into mm/mlock.c has helped: mlock_new_page(), using the mlock pagevec,
> rather than lru_cache_add()'s pagevec.
>
> Experimented with various orderings: the right thing seems to be for
> mlock_page() and mlock_new_page() to TestSetPageMlocked before adding to
> pagevec, but munlock_page() to leave TestClearPageMlocked to the later
> pagevec processing.
>
> Dropped the VM_BUG_ON_PAGE(PageTail)s this time around: they have made
> their point, and the thp_nr_page()s already contain a VM_BUG_ON_PGFLAGS()
> for that.
>
> This still leaves acquiring lruvec locks under page table lock each time
> the pagevec fills (or a THP is added): which I suppose is rather silly,
> since they sit on pagevec waiting to be processed long after page table
> lock has been dropped; but I'm disinclined to uglify the calling sequence
> until some load shows an actual problem with it (nothing wrong with
> taking lruvec lock under page table lock, just "nicer" to do it less).
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>

Thanks for your patch, which is now commit cbaf47432c909044
("mm/munlock: mlock_page() munlock_page() batch by pagevec") in
next-20220209.

> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -402,7 +402,8 @@ extern int mlock_future_check(struct mm_struct *mm, unsigned long flags,
>   *
>   * mlock is usually called at the end of page_add_*_rmap(),
>   * munlock at the end of page_remove_rmap(); but new anon
> - * pages are managed in lru_cache_add_inactive_or_unevictable().
> + * pages are managed by lru_cache_add_inactive_or_unevictable()
> + * calling mlock_new_page().
>   *
>   * @compound is used to include pmd mappings of THPs, but filter out
>   * pte mappings of THPs, which cannot be consistently counted: a pte
> @@ -425,6 +426,9 @@ static inline void munlock_vma_page(struct page *page,
>             (compound || !PageTransCompound(page)))
>                 munlock_page(page);
>  }
> +void mlock_new_page(struct page *page);
> +bool need_mlock_page_drain(int cpu);
> +void mlock_page_drain(int cpu);

This is inside an #ifdef CONFIG_MMU section.

> --- a/mm/swap.c
> +++ b/mm/swap.c

> @@ -640,6 +634,7 @@ void lru_add_drain_cpu(int cpu)
>                 pagevec_lru_move_fn(pvec, lru_lazyfree_fn);
>
>         activate_page_drain(cpu);
> +       mlock_page_drain(cpu);

noreply@xxxxxxxxxxxxxx reported for m5272c3_defconfig:

    mm/swap.c:637:2: error: implicit declaration of function
‘mlock_page_drain’ [-Werror=implicit-function-declaration]

http://kisskb.ellerman.id.au/kisskb/buildresult/14694567/

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds