From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
"Al Viro" <viro@zeniv.linux.org.uk>,
"Alex Williamson" <alex.williamson@redhat.com>,
"Benjamin Herrenschmidt" <benh@kernel.crashing.org>,
"Björn Töpel" <bjorn.topel@intel.com>,
"Christoph Hellwig" <hch@infradead.org>,
"Dan Williams" <dan.j.williams@intel.com>,
"Daniel Vetter" <daniel@ffwll.ch>,
"Dave Chinner" <david@fromorbit.com>,
"David Airlie" <airlied@linux.ie>,
"David S . Miller" <davem@davemloft.net>,
"Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>,
"Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>,
"Jonathan Corbet" <corbet@lwn.net>,
"Jérôme Glisse" <jglisse@redhat.com>,
"Magnus Karlsson" <magnus.karlsson@intel.com>,
"Mauro Carvalho Chehab" <mchehab@kernel.org>,
"Michael Ellerman" <mpe@ellerman.id.au>,
"Michal Hocko" <mhocko@suse.com>,
"Mike Kravetz" <mike.kravetz@oracle.com>,
"Paul Mackerras" <paulus@samba.org>,
"Shuah Khan" <shuah@kernel.org>,
"Vlastimil Babka" <vbabka@suse.cz>,
bpf@vger.kernel.org, dri-devel@lists.freedesktop.org,
kvm@vger.kernel.org, linux-block@vger.kernel.org,
linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org,
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netdev@vger.kernel.org, linux-mm@kvack.org,
LKML <linux-kernel@vger.kernel.org>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v9 23/25] mm/gup: track FOLL_PIN pages
Date: Wed, 11 Dec 2019 12:28:07 +0100 [thread overview]
Message-ID: <20191211112807.GN1551@quack2.suse.cz> (raw)
In-Reply-To: <20191211025318.457113-24-jhubbard@nvidia.com>
On Tue 10-12-19 18:53:16, John Hubbard wrote:
> Add tracking of pages that were pinned via FOLL_PIN.
>
> As mentioned in the FOLL_PIN documentation, callers who effectively set
> FOLL_PIN are required to ultimately free such pages via unpin_user_page().
> The effect is similar to FOLL_GET, and may be thought of as "FOLL_GET
> for DIO and/or RDMA use".
>
> Pages that have been pinned via FOLL_PIN are identifiable via a
> new function call:
>
> bool page_dma_pinned(struct page *page);
>
> What to do in response to encountering such a page, is left to later
> patchsets. There is discussion about this in [1], [2], and [3].
>
> This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
>
> [1] Some slow progress on get_user_pages() (Apr 2, 2019):
> https://lwn.net/Articles/784574/
> [2] DMA and get_user_pages() (LPC: Dec 12, 2018):
> https://lwn.net/Articles/774411/
> [3] The trouble with get_user_pages() (Apr 30, 2018):
> https://lwn.net/Articles/753027/
The patch looks mostly good to me now. Just a few smaller comments below.
> Suggested-by: Jan Kara <jack@suse.cz>
> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
I think you inherited here the Reviewed-by tags from the "add flags" patch
you've merged into this one but that's not really fair since this patch
does much more... In particular I didn't give my Reviewed-by tag for this
patch yet.
> +/*
> + * try_grab_compound_head() - attempt to elevate a page's refcount, by a
> + * flags-dependent amount.
> + *
> + * This has a default assumption of "use FOLL_GET behavior, if FOLL_PIN is not
> + * set".
> + *
> + * "grab" names in this file mean, "look at flags to decide whether to use
> + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
> + */
> +static __maybe_unused struct page *try_grab_compound_head(struct page *page,
> + int refs,
> + unsigned int flags)
> +{
> + if (flags & FOLL_PIN)
> + return try_pin_compound_head(page, refs);
> +
> + return try_get_compound_head(page, refs);
> +}
I somewhat wonder about the asymmetry of try_grab_compound_head() vs
try_grab_page() in the treatment of 'flags'. How costly would it be to make
them symmetric (i.e., either set FOLL_GET for try_grab_compound_head()
callers or make sure one of FOLL_GET, FOLL_PIN is set for try_grab_page())?
Because this difference looks like a subtle catch in the long run...
> +
> +/**
> + * try_grab_page() - elevate a page's refcount by a flag-dependent amount
> + *
> + * This might not do anything at all, depending on the flags argument.
> + *
> + * "grab" names in this file mean, "look at flags to decide whether to use
> + * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
> + *
> + * @page: pointer to page to be grabbed
> + * @flags: gup flags: these are the FOLL_* flag values.
> + *
> + * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
> + * time. (That's true throughout the get_user_pages*() and pin_user_pages*()
> + * APIs.) Cases:
> + *
> + * FOLL_GET: page's refcount will be incremented by 1.
> + * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
> + *
> + * Return: true for success, or if no action was required (if neither FOLL_PIN
> + * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
> + * FOLL_PIN was set, but the page could not be grabbed.
> + */
> +bool __must_check try_grab_page(struct page *page, unsigned int flags)
> +{
> + if (flags & FOLL_GET)
> + return try_get_page(page);
> + else if (flags & FOLL_PIN) {
> + page = compound_head(page);
> + WARN_ON_ONCE(flags & FOLL_GET);
> +
> + if (WARN_ON_ONCE(page_ref_zero_or_close_to_bias_overflow(page)))
> + return false;
> +
> + page_ref_add(page, GUP_PIN_COUNTING_BIAS);
> + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
> + }
> +
> + return true;
> +}
...
> @@ -1522,8 +1536,8 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> skip_mlock:
> page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
> VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
> - if (flags & FOLL_GET)
> - get_page(page);
> + if (!try_grab_page(page, flags))
> + page = ERR_PTR(-EFAULT);
I think you need to also move the try_grab_page() earlier in the function.
At this point the page may be marked as mlocked and you'd need to undo that
in case try_grab_page() fails.
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index ac65bb5e38ac..0aab6fe0072f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4356,7 +4356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> same_page:
> if (pages) {
> pages[i] = mem_map_offset(page, pfn_offset);
> - get_page(pages[i]);
> + if (!try_grab_page(pages[i], flags)) {
> + spin_unlock(ptl);
> + remainder = 0;
> + err = -ENOMEM;
> + WARN_ON_ONCE(1);
> + break;
> + }
> }
This function does a refcount overflow check early so that it doesn't have
to do try_get_page() here. So that check can be now removed when you do
try_grab_page() here anyway since that early check seems to be just a tiny
optimization AFAICT.
Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2019-12-11 11:28 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-11 2:52 [PATCH v9 00/25] mm/gup: track dma-pinned pages: FOLL_PIN John Hubbard
2019-12-11 2:52 ` [PATCH v9 01/25] mm/gup: factor out duplicate code from four routines John Hubbard
2019-12-11 2:52 ` [PATCH v9 02/25] mm/gup: move try_get_compound_head() to top, fix minor issues John Hubbard
2019-12-11 2:52 ` [PATCH v9 03/25] mm: Cleanup __put_devmap_managed_page() vs ->page_free() John Hubbard
2019-12-11 2:52 ` [PATCH v9 04/25] mm: devmap: refactor 1-based refcounting for ZONE_DEVICE pages John Hubbard
2019-12-11 2:52 ` [PATCH v9 05/25] goldish_pipe: rename local pin_user_pages() routine John Hubbard
2019-12-11 2:52 ` [PATCH v9 06/25] mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM John Hubbard
2019-12-11 2:53 ` [PATCH v9 07/25] vfio: fix FOLL_LONGTERM use, simplify get_user_pages_remote() call John Hubbard
2019-12-11 2:53 ` [PATCH v9 08/25] mm/gup: allow FOLL_FORCE for get_user_pages_fast() John Hubbard
2019-12-11 2:53 ` [PATCH v9 09/25] IB/umem: use get_user_pages_fast() to pin DMA pages John Hubbard
2019-12-11 2:53 ` [PATCH v9 10/25] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard
2019-12-11 20:57 ` Jonathan Corbet
2019-12-11 21:25 ` John Hubbard
2019-12-11 2:53 ` [PATCH v9 11/25] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard
2019-12-11 2:53 ` [PATCH v9 12/25] IB/{core,hw,umem}: set FOLL_PIN via pin_user_pages*(), fix up ODP John Hubbard
2019-12-11 2:53 ` [PATCH v9 13/25] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard
2019-12-11 2:53 ` [PATCH v9 14/25] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard
2019-12-11 2:53 ` [PATCH v9 15/25] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard
2019-12-11 2:53 ` [PATCH v9 16/25] net/xdp: " John Hubbard
2019-12-11 2:53 ` [PATCH v9 17/25] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard
2019-12-11 2:53 ` [PATCH v9 18/25] media/v4l2-core: pin_user_pages (FOLL_PIN) and put_user_page() conversion John Hubbard
2019-12-11 2:53 ` [PATCH v9 19/25] vfio, mm: " John Hubbard
2019-12-11 2:53 ` [PATCH v9 20/25] powerpc: book3s64: convert to pin_user_pages() and put_user_page() John Hubbard
2019-12-11 10:42 ` Jan Kara
2019-12-11 2:53 ` [PATCH v9 21/25] mm/gup_benchmark: use proper FOLL_WRITE flags instead of hard-coding "1" John Hubbard
2019-12-11 2:53 ` [PATCH v9 22/25] mm, tree-wide: rename put_user_page*() to unpin_user_page*() John Hubbard
2019-12-11 2:53 ` [PATCH v9 23/25] mm/gup: track FOLL_PIN pages John Hubbard
2019-12-11 11:28 ` Jan Kara [this message]
2019-12-12 5:53 ` John Hubbard
2019-12-17 8:03 ` kbuild test robot
2019-12-17 8:03 ` [RFC PATCH] mm/gup: try_pin_compound_head() can be static kbuild test robot
2019-12-17 13:56 ` John Hubbard
2019-12-11 2:53 ` [PATCH v9 24/25] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard
2019-12-11 2:53 ` [PATCH v9 25/25] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard
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