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. > > 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. Thanks! -- Michal Hocko SUSE Labs
