On Mon, Jan 10, 2022 at 03:37:28PM +0100, Michal Hocko wrote: > On Sun 09-01-22 20:58:02, Yu Zhao wrote: > > On Fri, Jan 07, 2022 at 10:00:31AM +0100, Michal Hocko wrote: > > > On Fri 07-01-22 09:55:09, Michal Hocko wrote: > > > [...] > > > > > In this case, lru_gen_mm_walk is small (160 bytes); it's per direct > > > > > reclaimer; and direct reclaimers rarely come here, i.e., only when > > > > > kswapd can't keep up in terms of the aging, which is similar to the > > > > > condition where the inactive list is empty for the active/inactive > > > > > lru. > > > > > > > > Well, this is not a strong argument to be honest. Kswapd being stuck > > > > and the majority of the reclaim being done in the direct reclaim > > > > context is a situation I have seen many many times. > > > > > > Also do not forget that memcg reclaim is effectivelly only direct > > > reclaim. Not that the memcg reclaim indicates a global memory shortage > > > but it can add up and race with the global reclaim as well. > > > > I don't dispute any of the above, and I probably don't like this code > > more than you do. > > > > But let's not forget the purposes of PF_MEMALLOC, besides preventing > > recursive reclaims, include letting reclaim dip into reserves so that > > it can make more free memory. So I think it's acceptable if the > > following conditions are met: > > 1. The allocation size is small. > > 2. The number of allocations is bounded. > > 3. Its failure doesn't stall reclaim. > > And it'd be nice if > > 4. The allocation happens rarely, e.g., slow path only. > > I would add > 0. The allocation should be done only if absolutely _necessary_. > > Please keep in mind that whatever you allocate from that context will be > consuming a very precious memory reserves which are shared with other > components of the system. Even worse these can go all the way to > depleting memory completely where other things can fall apart. I agree but I also see a distinction: 1,2,3 are objective; 0,4 are subjective. For some users, page reclaim itself could be not absolutely necessary because they are okay with OOM kills. But for others, the situation could be reversed. > > The code in question meets all of them. > > > > 1. This allocation is 160 bytes. > > 2. It's bounded by the number of page table walkers which, in the > > worst, is same as the number of mm_struct's. > > 3. Most importantly, its failure doesn't stall the aging. The aging > > will fallback to the rmap-based function lru_gen_look_around(). > > But this function only gathers the accessed bit from at most 64 > > PTEs, meaning it's less efficient (retains ~80% performance gains). > > 4. This allocation is rare, i.e., only when the aging is required, > > which is similar to the low inactive case for the active/inactive > > lru. > > I think this fallback behavior deserves much more detailed explanation > in changelogs. Will do. > > The bottom line is I can try various optimizations, e.g., preallocate > > a few buffers for a limited number of page walkers and if this number > > has been reached, fallback to the rmap-based function. But I have yet > > to see evidence that calls for additional complexity. > > I would disagree here. This is not an optimization. You should be > avoiding allocations from the memory reclaim because any allocation just > add a runtime behavior complexity and potential corner cases. Would __GFP_NOMEMALLOC address your concern? It prevents allocations from accessing the reserves even under PF_MEMALLOC.
