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Shutemov" To: Yang Shi Cc: mhocko@kernel.org, willy@infradead.org, ldufour@linux.vnet.ibm.com, vbabka@suse.cz, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [v2 PATCH 1/2 -mm] mm: mremap: dwongrade mmap_sem to read when shrinking Message-ID: <20180927124618.dsg4xtxcmn5hrdj6@kshutemo-mobl1> References: <1537985434-22655-1-git-send-email-yang.shi@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1537985434-22655-1-git-send-email-yang.shi@linux.alibaba.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 27, 2018 at 02:10:33AM +0800, Yang Shi wrote: > Other than munmap, mremap might be used to shrink memory mapping too. > So, it may hold write mmap_sem for long time when shrinking large > mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in > munmap") described. > > The mremap() will not manipulate vmas anymore after __do_munmap() call for > the mapping shrink use case, so it is safe to downgrade to read mmap_sem. > > So, the same optimization, which downgrades mmap_sem to read for zapping > pages, is also feasible and reasonable to this case. > > The period of holding exclusive mmap_sem for shrinking large mapping > would be reduced significantly with this optimization. > > MREMAP_FIXED and MREMAP_MAYMOVE are more complicated to adopt this > optimization since they need manipulate vmas after do_munmap(), > downgrading mmap_sem may create race window. > > Simple mapping shrink is the low hanging fruit, and it may cover the > most cases of unmap with munmap together. > > Cc: Michal Hocko > Cc: Kirill A. Shutemov > Cc: Matthew Wilcox > Cc: Laurent Dufour > Cc: Vlastimil Babka > Cc: Andrew Morton > Signed-off-by: Yang Shi > --- > v2: Rephrase the commit log per Michal > > include/linux/mm.h | 2 ++ > mm/mmap.c | 4 ++-- > mm/mremap.c | 17 +++++++++++++---- > 3 files changed, 17 insertions(+), 6 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index a61ebe8..3028028 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2286,6 +2286,8 @@ extern unsigned long do_mmap(struct file *file, unsigned long addr, > unsigned long len, unsigned long prot, unsigned long flags, > vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate, > struct list_head *uf); > +extern int __do_munmap(struct mm_struct *, unsigned long, size_t, > + struct list_head *uf, bool downgrade); > extern int do_munmap(struct mm_struct *, unsigned long, size_t, > struct list_head *uf); > > diff --git a/mm/mmap.c b/mm/mmap.c > index 847a17d..017bcfa 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -2687,8 +2687,8 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma, > * work. This now handles partial unmappings. > * Jeremy Fitzhardinge > */ > -static int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len, > - struct list_head *uf, bool downgrade) > +int __do_munmap(struct mm_struct *mm, unsigned long start, size_t len, > + struct list_head *uf, bool downgrade) > { > unsigned long end; > struct vm_area_struct *vma, *prev, *last; > diff --git a/mm/mremap.c b/mm/mremap.c > index 5c2e185..8f1ec2b 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -525,6 +525,7 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta) > unsigned long ret = -EINVAL; > unsigned long charged = 0; > bool locked = false; > + bool downgrade = false; s/downgrade/downgraded/ ? Otherwise looks good to me: Acked-by: Kirill A. Shutemov -- Kirill A. Shutemov