Re: [PATCH v2] mm: memory: move mem_cgroup_charge() into alloc_anon_folio()

[Kefeng Wang]

On 2024/1/19 16:00, Michal Hocko wrote:
> On Fri 19-01-24 10:05:15, Kefeng Wang wrote:
> > On 2024/1/18 23:59, Michal Hocko wrote:
> > > On Wed 17-01-24 18:39:54, Kefeng Wang wrote:
> > > > mem_cgroup_charge() uses the GFP flags in a fairly sophisticated way.
> > > > In addition to checking gfpflags_allow_blocking(), it pays attention
> > > > to __GFP_NORETRY and __GFP_RETRY_MAYFAIL to ensure that processes within
> > > > this memcg do not exceed their quotas. Using the same GFP flags ensures
> > > > that we handle large anonymous folios correctly, including falling back
> > > > to smaller orders when there is plenty of memory available in the system
> > > > but this memcg is close to its limits.
> > >
> > > The changelog is not really clear in the actual problem you are trying
> > > to fix. Is this pure consistency fix or have you actually seen any
> > > misbehavior. From the patch I suspect you are interested in THPs much
> > > more than regular order-0 pages because those are GFP_KERNEL like when
> > > it comes to charging. THPs have a variety of options on how aggressive
> > > the allocation should try. From that perspective NORETRY and
> > > RETRY_MAYFAIL are not all that interesting because costly allocations
> > > (which THPs are) already do imply MAYFAIL and NORETRY.
> >
> > I don't meet actual issue, it founds from code inspection.
> >
> > mTHP is introduced by Ryan（19eaf44954df "mm: thp: support allocation of
> > anonymous multi-size THP")，so we have similar check for mTHP like PMD THP
> > in alloc_anon_folio(), it will try to allocate large order folio below
> > PMD_ORDER, and fallback to order-0 folio if fails, meanwhile,
> > it get GFP flags from vma_thp_gfp_mask() according to user configuration
> > like PMD THP allocation, so
> >
> > 1) the memory charge failure check should be moved into fallback
> > logical, because it will make us to allocated as much as possible large
> > order folio, although the memcg's memory usage is close to its limits.
> >
> > 2) using seem GFP flags for allocate/mem charge, be consistent with PMD
> > THP firstly, in addition, according to GFP flag returned for
> > vma_thp_gfp_mask(), GFP_TRANSHUGE_LIGHT could make us skip direct reclaim,
> > _GFP_NORETRY will make us skip mem_cgroup_oom and won't kill
> > any progress from large order folio charging.
>
> OK, makes sense. Please turn that into the changelog.

Sure.

> > > GFP_TRANSHUGE_LIGHT is more interesting though because those do not dive
> > > into the direct reclaim at all. With the current code they will reclaim
> > > charges to free up the space for the allocated THP page and that defeats
> > > the light mode. I have a vague recollection of preparing a patch to
> >
> > We are interesting to GFP_TRANSHUGE_LIGHT and _GFP_NORETRY as mentioned
> > above.
>
> if mTHP can be smaller than COSTLY_ORDER then you are correct and
> NORETRY makes a difference. Please mention that in the changelog as
> well.

For memory cgroup charge, _GFP_NORETRY checked to make us directly skip
mem_cgroup_oom(), it has no concern with folio order or COSTLY_ORDER 
when check _GFP_NORETRY in try_charge_memcg(), so I think NORETRY should
always make difference for all large order folio.