From: Jason Gunthorpe <jgg@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: John Hubbard <jhubbard@nvidia.com>,
Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-rdma@vger.kernel.org, linux-mm@kvack.org,
Mike Marciniszyn <mike.marciniszyn@intel.com>,
Leon Romanovsky <leon@kernel.org>,
Artemy Kovalyov <artemyko@nvidia.com>,
Michael Guralnik <michaelgur@nvidia.com>,
Alistair Popple <apopple@nvidia.com>,
Pak Markthub <pmarkthub@nvidia.com>
Subject: Re: [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches
Date: Wed, 1 May 2024 09:10:32 -0300 [thread overview]
Message-ID: <20240501121032.GA941030@nvidia.com> (raw)
In-Reply-To: <ZjHO04Rb75TIlmkA@infradead.org>
On Tue, Apr 30, 2024 at 10:10:43PM -0700, Christoph Hellwig wrote:
> > + pinned = -ENOMEM;
> > + int attempts = 0;
> > + /*
> > + * pin_user_pages_fast() can return -EAGAIN, due to falling back
> > + * to gup-slow and then failing to migrate pages out of
> > + * ZONE_MOVABLE due to a transient elevated page refcount.
> > + *
> > + * One retry is enough to avoid this problem, so far, but let's
> > + * use a slightly higher retry count just in case even larger
> > + * systems have a longer-lasting transient refcount problem.
> > + *
> > + */
> > + static const int MAX_ATTEMPTS = 3;
> > +
> > + while (pinned == -EAGAIN && attempts < MAX_ATTEMPTS) {
> > + pinned = pin_user_pages_fast(cur_base,
> > + min_t(unsigned long,
> > + npages, PAGE_SIZE /
> > + sizeof(struct page *)),
> > + gup_flags, page_list);
> > ret = pinned;
> > - goto umem_release;
> > + attempts++;
> > +
> > + if (pinned == -EAGAIN)
> > + continue;
> > }
> > + if (pinned < 0)
> > + goto umem_release;
>
> This doesn't make sense. IFF a blind retry is all that is needed it
> should be done in the core functionality. I fear it's not that easy,
> though.
+1
This migration retry weirdness is a GUP issue, it needs to be solved
in the mm not exposed to every pin_user_pages caller.
If it turns out ZONE_MOVEABLE pages can't actually be reliably moved
then it is pretty broken..
Jason
next prev parent reply other threads:[~2024-05-01 12:10 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 0:31 [RFC] RDMA/umem: pin_user_pages*() can temporarily fail due to migration glitches John Hubbard
2024-05-01 5:10 ` Christoph Hellwig
2024-05-01 12:10 ` Jason Gunthorpe [this message]
2024-05-01 17:32 ` John Hubbard
2024-05-02 1:05 ` Alistair Popple
2024-05-02 6:49 ` John Hubbard
2024-05-02 6:56 ` David Hildenbrand
2024-05-02 18:10 ` John Hubbard
2024-05-02 18:34 ` Jason Gunthorpe
2024-05-02 18:44 ` Matthew Wilcox
2024-05-10 7:54 ` David Hildenbrand
2024-05-11 0:32 ` Kasireddy, Vivek
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