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.
