Re: [PATCH mm-unstable v1 7/8] mm: multi-gen LRU: clarify scan_control flags

[Hillf Danton <hdanton@sina.com>]

On 1 Dec 2022 15:39:23 -0700 Yu Zhao <yuzhao@google.com>
> Among the flags in scan_control:
> 1. sc->may_swap, which indicates swap constraint due to memsw.max, is
>    supported as usual.
> 2. sc->proactive, which indicates reclaim by memory.reclaim, may not
>    opportunistically skip the aging path, since it is considered less
>    latency sensitive.
> 3. !(sc->gfp_mask & __GFP_IO), which indicates IO constraint,
>    prioritizes file LRU, since clean file folios are more likely to
>    exist.
> 4. sc->may_writepage and sc->may_unmap, which indicates opportunistic
>    reclaim, are rejected, since unmapped clean folios are already
>    prioritized. Scanning for more of them is likely futile and can
>    cause high reclaim latency when there is a large number of memcgs.

Nit, just because of gfp without __GFP_IO set does not mean there are
likely more clean page caches, though prioritized, on the local numa node
than a remote one, and vice verse.

Hillf

/**
 * memalloc_noio_save - Marks implicit GFP_NOIO allocation scope.
 *
 * This functions marks the beginning of the GFP_NOIO allocation scope.
 * All further allocations will implicitly drop __GFP_IO flag and so
 * they are safe for the IO critical section from the allocation recursion
 * point of view. Use memalloc_noio_restore to end the scope with flags
 * returned by this function.
 *
 * This function is safe to be used from any context.
 */