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Mon, 14 Sep 2020 05:31:33 -0400 X-MC-Unique: n1VGXgdVP0mLvQD--89XHQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7A4541882FA1; Mon, 14 Sep 2020 09:31:31 +0000 (UTC) Received: from [10.36.114.162] (ovpn-114-162.ams2.redhat.com [10.36.114.162]) by smtp.corp.redhat.com (Postfix) with ESMTP id C63585DA60; Mon, 14 Sep 2020 09:31:24 +0000 (UTC) Subject: Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc To: Chris Goldsworthy , akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, pratikp@codeaurora.org, pdaly@codeaurora.org, sudraja@codeaurora.org, iamjoonsoo.kim@lge.com, linux-arm-msm-owner@vger.kernel.org, Vinayak Menon References: <06489716814387e7f147cf53d1b185a8@codeaurora.org> <1599851809-4342-1-git-send-email-cgoldswo@codeaurora.org> <010101747e998731-e49f209f-8232-4496-a9fc-2465334e70d7-000000@us-west-2.amazonses.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: <010101747e998731-e49f209f-8232-4496-a9fc-2465334e70d7-000000@us-west-2.amazonses.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11.09.20 21:17, Chris Goldsworthy wrote: > CMA allocations will fail if 'pinned' pages are in a CMA area, since we > cannot migrate pinned pages. The _refcount of a struct page being greater > than _mapcount for that page can cause pinning for anonymous pages. This > is because try_to_unmap(), which (1) is called in the CMA allocation path, > and (2) decrements both _refcount and _mapcount for a page, will stop > unmapping a page from VMAs once the _mapcount for a page reaches 0. This > implies that after try_to_unmap() has finished successfully for a page > where _recount > _mapcount, that _refcount will be greater than 0. Later > in the CMA allocation path in migrate_page_move_mapping(), we will have one > more reference count than intended for anonymous pages, meaning the > allocation will fail for that page. > > One example of where _refcount can be greater than _mapcount for a page we > would not expect to be pinned is inside of copy_one_pte(), which is called > during a fork. For ptes for which pte_present(pte) == true, copy_one_pte() > will increment the _refcount field followed by the _mapcount field of a > page. If the process doing copy_one_pte() is context switched out after > incrementing _refcount but before incrementing _mapcount, then the page > will be temporarily pinned. > > So, inside of cma_alloc(), instead of giving up when alloc_contig_range() > returns -EBUSY after having scanned a whole CMA-region bitmap, perform > retries indefinitely, with sleeps, to give the system an opportunity to > unpin any pinned pages. > > Signed-off-by: Chris Goldsworthy > Co-developed-by: Vinayak Menon > Signed-off-by: Vinayak Menon > --- > mm/cma.c | 25 +++++++++++++++++++++++-- > 1 file changed, 23 insertions(+), 2 deletions(-) > > diff --git a/mm/cma.c b/mm/cma.c > index 7f415d7..90bb505 100644 > --- a/mm/cma.c > +++ b/mm/cma.c > @@ -32,6 +32,7 @@ > #include > #include > #include > +#include > #include > > #include "cma.h" > @@ -442,8 +443,28 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align, > bitmap_maxno, start, bitmap_count, mask, > offset); > if (bitmap_no >= bitmap_maxno) { > - mutex_unlock(&cma->lock); > - break; > + if (ret == -EBUSY) { > + mutex_unlock(&cma->lock); > + > + /* > + * Page may be momentarily pinned by some other > + * process which has been scheduled out, e.g. > + * in exit path, during unmap call, or process > + * fork and so cannot be freed there. Sleep > + * for 100ms and retry the allocation. > + */ > + start = 0; > + ret = -ENOMEM; > + msleep(100); > + continue; > + } else { > + /* > + * ret == -ENOMEM - all bits in cma->bitmap are > + * set, so we break accordingly. > + */ > + mutex_unlock(&cma->lock); > + break; > + } > } > bitmap_set(cma->bitmap, bitmap_no, bitmap_count); > /* > What about long-term pinnings? IIRC, that can happen easily e.g., with vfio (and I remember there is a way via vmsplice). Not convinced trying forever is a sane approach in the general case ... -- Thanks, David / dhildenb