On Thu, Jan 13, 2022 at 01:02:26PM +0100, Michal Hocko wrote: > On Thu 13-01-22 02:43:38, Yu Zhao wrote: > [...] > > > > 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. > > __GFP_NOMEMALLOC would deal with the complete memory depletion concern > for sure but I am not sure how any of these allocations would succeed > when called from the direct reclaim. Some access to memory reserves is > necessary if you insist on allocating from the reclaim process. > > You can have a look at the limited memory reserves access by oom victims > for an example of how this can be done. Thanks. I'll change GFP_KERNEL to __GFP_HIGH | __GFP_NOMEMALLOC. __GFP_HIGH allows some access to memory reserves and __GFP_NOMEMALLOC prevents the complete depletion. Basically the combination lower the min watermark by 1/2, and we have been using them for add_to_swap_cache().
