Re: [PATCH V3] mm: compaction: skip memory compaction when there are not enough migratable pages

[Ge Yang <yangge1116@126.com>]

在 2025/1/14 20:51, Vlastimil Babka 写道:
> On 1/14/25 13:24, Ge Yang wrote:
>>> Hopefully also when done from the pin_user_pages_remote(..., FOLL_LONGTERM,
>>> ...) context the allocation gfp_mask correctly lacks __GFP_MOVABLE?
>> yes.
>> I guess
>>> it has to, otherwise it would allocate from the CMA pageblocks.
>>>
>>> Then I wonder if we could use the real allocation context to determine
>>> watermarks, as __compaction_suitable() is passing ALLOC_CMA instead because
>>> it's checking only for migration targets, which have to be CMA compatible by
>>> definition. But we could use the real unmovable allocation context to have
>>> __zone_watermark_unusable_free() subtract CMA pages, and thus we won't pass
>>> the order-0 check anymore once the non-CMA part is exhausted.
>>>
>>> There's some risk that in some different scenario the compaction could in
>>> fact migrate pages from the exhausted non-CMA part of the zone to the CMA
>>> part and succeed, and we'll skip it instead. But that should be rare?
>>>
>> Below is the previous discussion：
>> https://lore.kernel.org/lkml/1734436004-1212-1-git-send-email-yangge1116@126.com/
> 
> Right so Johannes had the same concern.
> 
>>> Anyway given that concern I'm not sure about changing
>>> __compaction_suitable() for every caller like this. We could (at least
>>> initially) target this heuristic only for COMPACT_PRIO_ASYNC which is being
>>> used for this THP opportunistic attempt.
>>>
>>> So for example:
>>> - add a new bool flag to compact_control that is true for COMPACT_PRIO_ASYNC
>>> - pass cc pointer to compaction_suit_allocation_order()
>>> - in that function, add another check if the the new cc flag is true,
>>> between the current zone_watermark_ok() and compaction_suitable() checks,
>>> which works like __compaction_suitable() but uses alloc_flags (which should
>>> not be ALLOC_CMA in our pinned allocation case) instead of ALLOC_CMA, return
>>> COMPACT_SKIPPED if it fails.
>>>
>> I will send a new version of the patch based on the suggestions here.
>> Thank you.
> 
> Yeah that way should hopefully limit the concern sufficiently. Maybe we
> could also add costly_order condition in addition to COMPACT_PRIO_ASYNC
> condition to set the new compact_control flag. But only __GFP_NORETRY
> allocations should be affected in the immediate "goto nopage" when
> compaction is skipped, others will attempt with DEF_COMPACT_PRIORITY anyway
> and won't fail without trying to compact-migrate the non-CMA pageblocks into
> CMA pageblocks first, so it should be fine.
Ok, thanks.