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Message-ID: <20191031183549.GC14771@iweiny-DESK2.sc.intel.com>
Date: Thu, 31 Oct 2019 11:35:49 -0700
From: Ira Weiny <ira.weiny@...el.com>
To: John Hubbard <jhubbard@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Alex Williamson <alex.williamson@...hat.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Björn Töpel <bjorn.topel@...el.com>,
Christoph Hellwig <hch@...radead.org>,
Dan Williams <dan.j.williams@...el.com>,
Daniel Vetter <daniel@...ll.ch>,
Dave Chinner <david@...morbit.com>,
David Airlie <airlied@...ux.ie>,
"David S . Miller" <davem@...emloft.net>, Jan Kara <jack@...e.cz>,
Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>,
Jonathan Corbet <corbet@....net>,
Jérôme Glisse <jglisse@...hat.com>,
Magnus Karlsson <magnus.karlsson@...el.com>,
Mauro Carvalho Chehab <mchehab@...nel.org>,
Michael Ellerman <mpe@...erman.id.au>,
Michal Hocko <mhocko@...e.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Paul Mackerras <paulus@...ba.org>,
Shuah Khan <shuah@...nel.org>,
Vlastimil Babka <vbabka@...e.cz>, bpf@...r.kernel.org,
dri-devel@...ts.freedesktop.org, kvm@...r.kernel.org,
linux-block@...r.kernel.org, linux-doc@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-media@...r.kernel.org, linux-rdma@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
Christoph Hellwig <hch@....de>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>
Subject: Re: [PATCH 02/19] mm/gup: factor out duplicate code from four
routines
On Wed, Oct 30, 2019 at 03:49:13PM -0700, John Hubbard wrote:
> There are four locations in gup.c that have a fair amount of code
> duplication. This means that changing one requires making the same
> changes in four places, not to mention reading the same code four
> times, and wondering if there are subtle differences.
>
> Factor out the common code into static functions, thus reducing the
> overall line count and the code's complexity.
>
> Also, take the opportunity to slightly improve the efficiency of the
> error cases, by doing a mass subtraction of the refcount, surrounded
> by get_page()/put_page().
>
> Also, further simplify (slightly), by waiting until the the successful
> end of each routine, to increment *nr.
Overall it seems like a pretty good clean up. It did take a bit of review but
I _think_ it is correct. A couple of comments below.
>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> Signed-off-by: John Hubbard <jhubbard@...dia.com>
> ---
> mm/gup.c | 113 ++++++++++++++++++++++---------------------------------
> 1 file changed, 46 insertions(+), 67 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 85caf76b3012..8fb0d9cdfaf5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1969,6 +1969,35 @@ static int __gup_device_huge_pud(pud_t pud, pud_t *pudp, unsigned long addr,
> }
> #endif
>
> +static int __record_subpages(struct page *page, unsigned long addr,
> + unsigned long end, struct page **pages, int nr)
> +{
> + int nr_recorded_pages = 0;
> +
> + do {
> + pages[nr] = page;
> + nr++;
> + page++;
> + nr_recorded_pages++;
> + } while (addr += PAGE_SIZE, addr != end);
> + return nr_recorded_pages;
> +}
> +
> +static void __remove_refs_from_head(struct page *page, int refs)
> +{
> + /* Do a get_page() first, in case refs == page->_refcount */
> + get_page(page);
> + page_ref_sub(page, refs);
> + put_page(page);
> +}
I wonder if this is better implemented as "put_compound_head()"? To match the
try_get_compound_head() call below?
> +
> +static int __huge_pt_done(struct page *head, int nr_recorded_pages, int *nr)
> +{
> + *nr += nr_recorded_pages;
> + SetPageReferenced(head);
> + return 1;
When will this return anything but 1?
Ira
> +}
> +
> #ifdef CONFIG_ARCH_HAS_HUGEPD
> static unsigned long hugepte_addr_end(unsigned long addr, unsigned long end,
> unsigned long sz)
> @@ -1998,34 +2027,19 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
> /* hugepages are never "special" */
> VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>
> - refs = 0;
> head = pte_page(pte);
> -
> page = head + ((addr & (sz-1)) >> PAGE_SHIFT);
> - do {
> - VM_BUG_ON(compound_head(page) != head);
> - pages[*nr] = page;
> - (*nr)++;
> - page++;
> - refs++;
> - } while (addr += PAGE_SIZE, addr != end);
> + refs = __record_subpages(page, addr, end, pages, *nr);
>
> head = try_get_compound_head(head, refs);
> - if (!head) {
> - *nr -= refs;
> + if (!head)
> return 0;
> - }
>
> if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> - /* Could be optimized better */
> - *nr -= refs;
> - while (refs--)
> - put_page(head);
> + __remove_refs_from_head(head, refs);
> return 0;
> }
> -
> - SetPageReferenced(head);
> - return 1;
> + return __huge_pt_done(head, refs, nr);
> }
>
> static int gup_huge_pd(hugepd_t hugepd, unsigned long addr,
> @@ -2071,30 +2085,18 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> pages, nr);
> }
>
> - refs = 0;
> page = pmd_page(orig) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> - do {
> - pages[*nr] = page;
> - (*nr)++;
> - page++;
> - refs++;
> - } while (addr += PAGE_SIZE, addr != end);
> + refs = __record_subpages(page, addr, end, pages, *nr);
>
> head = try_get_compound_head(pmd_page(orig), refs);
> - if (!head) {
> - *nr -= refs;
> + if (!head)
> return 0;
> - }
>
> if (unlikely(pmd_val(orig) != pmd_val(*pmdp))) {
> - *nr -= refs;
> - while (refs--)
> - put_page(head);
> + __remove_refs_from_head(head, refs);
> return 0;
> }
> -
> - SetPageReferenced(head);
> - return 1;
> + return __huge_pt_done(head, refs, nr);
> }
>
> static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> @@ -2114,30 +2116,18 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> pages, nr);
> }
>
> - refs = 0;
> page = pud_page(orig) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
> - do {
> - pages[*nr] = page;
> - (*nr)++;
> - page++;
> - refs++;
> - } while (addr += PAGE_SIZE, addr != end);
> + refs = __record_subpages(page, addr, end, pages, *nr);
>
> head = try_get_compound_head(pud_page(orig), refs);
> - if (!head) {
> - *nr -= refs;
> + if (!head)
> return 0;
> - }
>
> if (unlikely(pud_val(orig) != pud_val(*pudp))) {
> - *nr -= refs;
> - while (refs--)
> - put_page(head);
> + __remove_refs_from_head(head, refs);
> return 0;
> }
> -
> - SetPageReferenced(head);
> - return 1;
> + return __huge_pt_done(head, refs, nr);
> }
>
> static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> @@ -2151,30 +2141,19 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> return 0;
>
> BUILD_BUG_ON(pgd_devmap(orig));
> - refs = 0;
> +
> page = pgd_page(orig) + ((addr & ~PGDIR_MASK) >> PAGE_SHIFT);
> - do {
> - pages[*nr] = page;
> - (*nr)++;
> - page++;
> - refs++;
> - } while (addr += PAGE_SIZE, addr != end);
> + refs = __record_subpages(page, addr, end, pages, *nr);
>
> head = try_get_compound_head(pgd_page(orig), refs);
> - if (!head) {
> - *nr -= refs;
> + if (!head)
> return 0;
> - }
>
> if (unlikely(pgd_val(orig) != pgd_val(*pgdp))) {
> - *nr -= refs;
> - while (refs--)
> - put_page(head);
> + __remove_refs_from_head(head, refs);
> return 0;
> }
> -
> - SetPageReferenced(head);
> - return 1;
> + return __huge_pt_done(head, refs, nr);
> }
>
> static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
> --
> 2.23.0
>
>
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