Newbie's question: memory allocation when reclaiming memory

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

May I ask a newbie question? Say, there is some amount of memory pages
which can be reclaimed if they are flushed to storage. And lower layer
might issue memory allocation request in a way which won't cause reclaim
deadlock (e.g. using GFP_NOFS or GFP_NOIO) when flushing to storage,
isn't it?

What I'm worrying is a dependency that __GFP_FS allocation requests think
that there are reclaimable pages and therefore there is no need to call
out_of_memory(); and GFP_NOFS allocation requests which the __GFP_FS
allocation requests depend on (in order to flush to storage) is waiting
for GFP_NOIO allocation requests; and the GFP_NOIO allocation requests
which the GFP_NOFS allocation requests depend on (in order to flush to
storage) are waiting for memory pages to be reclaimed without calling
out_of_memory(); because gfp_to_alloc_flags() does not favor GFP_NOIO over
GFP_NOFS nor GFP_NOFS over __GFP_FS which will throttle all allocations
at the same watermark level.

How do we guarantee that GFP_NOFS/GFP_NOIO allocations make forward
progress? What mechanism guarantees that memory pages which __GFP_FS
allocation requests are waiting for are reclaimed? I assume that there
is some mechanism; otherwise we can hit silent livelock, can't we?

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