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

Vlastimil Babka
2022-02-11T18:26:46Z

On 2/6/22 22:47, Hugh Dickins 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.

Not me, it seems.

> 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@google.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>