linux-kernel - Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV

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Message-ID: <20181029094253.GC16399@350D>
Date:   Mon, 29 Oct 2018 20:42:53 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Argangeli <andrea@...nel.org>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
        "Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Stable tree <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm: thp:  relax __GFP_THISNODE for MADV_HUGEPAGE
 mappings

On Mon, Oct 29, 2018 at 10:00:35AM +0100, Michal Hocko wrote:
> On Mon 29-10-18 16:17:52, Balbir Singh wrote:
> [...]
> > I wonder if alloc_pool_huge_page() should also trim out it's logic
> > of __GFP_THISNODE for the same reasons as mentioned here. I like
> > that we round robin to alloc the pool pages, but __GFP_THISNODE
> > might be an overkill for that case as well.
> 
> alloc_pool_huge_page uses __GFP_THISNODE for a different reason than
> THP. We really do want to allocated for a per-node pool. THP can
> fallback or use a different node.
> 

Not really

static int alloc_pool_huge_page(struct hstate *h, nodemask_t *nodes_allowed)
...
        gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;
...
        for_each_node_mask_to_alloc(h, nr_nodes, node, nodes_allowed) {
                page = alloc_fresh_huge_page(h, gfp_mask, node, nodes_allowed);
                if (page)
                        break;
        }


The code just tries to distribute the pool

> These hugetlb allocations might be disruptive and that is an expected
> behavior because this is an explicit requirement from an admin to
> pre-allocate large pages for the future use. __GFP_RETRY_MAYFAIL just
> underlines that requirement.

Yes, but in the absence of a particular node, for example via sysctl
(as the compaction does), I don't think it is a hard requirement to get
a page from a particular node. I agree we need __GFP_RETRY_FAIL, in any
case the real root cause for me is should_reclaim_continue() which keeps
the task looping without making forward progress.

The __GFP_THISNODE was again an example of mis-leading the allocator
in this case, IMHO.

> 
> Maybe the compaction logic could be improved and that might be a shared
> goal with future changes though.

I'll also send my RFC once my testing is done, assuming I get it to reproduce
with a desired frequency.

Balbir Singh.