Re: [PATCH mmotm] mm: alloc_large_system_hash check order

[Hugh Dickins <hugh@veritas.com>]

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On Fri, 1 May 2009, Eric Dumazet wrote:
> Hugh Dickins a écrit :
> > On Thu, 30 Apr 2009, Mel Gorman wrote:
> >> On Wed, Apr 29, 2009 at 10:09:48PM +0100, Hugh Dickins wrote:
> >>> On an x86_64 with 4GB ram, tcp_init()'s call to alloc_large_system_hash(),
> >>> to allocate tcp_hashinfo.ehash, is now triggering an mmotm WARN_ON_ONCE on
> >>> order >= MAX_ORDER - it's hoping for order 11.  alloc_large_system_hash()
> >>> had better make its own check on the order.
> 
> Well, I dont know why, since alloc_large_system_hash() already take
> care of retries, halving size between each tries.

Sorry, I wasn't clear: I just meant that if we keep that
WARN_ON_ONCE(order >= MAX_ORDER) in __alloc_pages_slowpath(),
then we need alloc_large_system_hash() to avoid the call to
__get_free_pages() in the order >= MAX_ORDER case,
precisely because we're happy with the way it halves and
falls back, so don't want a noisy warning; and now that we know
that it could give that warning, it would be a shame for the
_ONCE to suppress more interesting warnings later.

I certainly did not mean for alloc_large_system_hash() to fail
in the order >= MAX_ORDER case, nor did the patch do so.

Hugh