Re: [RFC PATCH 2/2] xfs: map KM_MAYFAIL to __GFP_RETRY_HARD
From: Johannes Weiner <hannes@cmpxchg.org>
Date: 2016-06-17 18:25:10
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lkml
On Thu, Jun 16, 2016 at 01:26:06PM +0200, Michal Hocko wrote:
quoted hunk ↗ jump to hunk
@@ -54,6 +54,13 @@ kmem_flags_convert(xfs_km_flags_t flags) lflags &= ~__GFP_FS; } + /* + * Default page/slab allocator behavior is to retry for ever + * for small allocations. We can override this behavior by using + * __GFP_RETRY_HARD which will tell the allocator to retry as long + * as it is feasible but rather fail than retry for ever for all + * request sizes. + */ if (flags & KM_MAYFAIL) lflags |= __GFP_RETRY_HARD;
I think this example shows that __GFP_RETRY_HARD is not a good flag because it conflates two seemingly unrelated semantics; the comment doesn't quite make up for that. When the flag is set, - it allows costly orders to invoke the OOM killer and retry - it allows !costly orders to fail While 1. is obvious from the name, 2. is not. Even if we don't want full-on fine-grained naming for every reclaim methodology and retry behavior, those two things just shouldn't be tied together. I don't see us failing !costly order per default anytime soon, and they are common, so adding a __GFP_MAYFAIL to explicitely override that behavior seems like a good idea to me. That would make the XFS callsite here perfectly obvious. And you can still combine it with __GFP_REPEAT. For a generic allocation site like this, __GFP_MAYFAIL | __GFP_REPEAT does the right thing for all orders, and it's self-explanatory: try hard, allow falling back. Whether we want a __GFP_REPEAT or __GFP_TRY_HARD at all is a different topic. In the long term, it might be better to provide best-effort per default and simply annotate MAYFAIL/NORETRY callsites that want to give up earlier. Because as I mentioned at LSFMM, it's much easier to identify callsites that have a convenient fallback than callsites that need to "try harder." Everybody thinks their allocations are oh so important. The former is much more specific and uses obvious criteria. Either way, __GFP_MAYFAIL should be on its own. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>