mm: brk: downgrade mmap_sem to read when shrinking (9bc8039e) · Commits · 戴 / test

brk might be used to shrink memory mapping too other than munmap(). 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 brk() will not manipulate vmas anymore after __do_munmap() call for the mapping shrink use case. But, it may set mm->brk after __do_munmap(), which needs hold write mmap_sem. However, a simple trick can workaround this by setting mm->brk before __do_munmap(). Then restore the original value if __do_munmap() fails. With this trick, 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. [akpm@linux-foundation.org: tweak comment] [yang.shi@linux.alibaba.com: fix unsigned compare against 0 issue] Link: http://lkml.kernel.org/r/1538687672-17795-1-git-send-email-yang.shi@linux.alibaba.com Link: http://lkml.kernel.org/r/1538067582-60038-2-git-send-email-yang.shi@linux.alibaba.com Signed-off-by:

Yang Shi <yang.shi@linux.alibaba.com> Acked-by:

Vlastimil Babka <vbabka@suse.cz> Acked-by:

Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com> Cc: Colin Ian King <colin.king@canonical.com> Signed-off-by:

Andrew Morton <akpm@linux-foundation.org> Signed-off-by:

Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

+35 −11

Original line number	Diff line number	Diff line
		@@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
		SYSCALL_DEFINE1(brk, unsigned long, brk)
		{
		unsigned long retval;
		unsigned long newbrk, oldbrk;
		unsigned long newbrk, oldbrk, origbrk;
		struct mm_struct *mm = current->mm;
		struct vm_area_struct *next;
		unsigned long min_brk;
		bool populate;
		bool downgraded = false;
		LIST_HEAD(uf);

		if (down_write_killable(&mm->mmap_sem))
		return -EINTR;

		origbrk = mm->brk;

		#ifdef CONFIG_COMPAT_BRK
		/*
		* CONFIG_COMPAT_BRK can still be overridden by setting
		@@ -229,14 +232,32 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)

		newbrk = PAGE_ALIGN(brk);
		oldbrk = PAGE_ALIGN(mm->brk);
		if (oldbrk == newbrk)
		goto set_brk;
		if (oldbrk == newbrk) {
		mm->brk = brk;
		goto success;
		}

		/* Always allow shrinking brk. */
		/*
		* Always allow shrinking brk.
		* __do_munmap() may downgrade mmap_sem to read.
		*/
		if (brk <= mm->brk) {
		if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf))
		goto set_brk;
		int ret;

		/*
		* mm->brk must to be protected by write mmap_sem so update it
		* before downgrading mmap_sem. When __do_munmap() fails,
		* mm->brk will be restored from origbrk.
		*/
		mm->brk = brk;
		ret = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true);
		if (ret < 0) {
		mm->brk = origbrk;
		goto out;
		} else if (ret == 1) {
		downgraded = true;
		}
		goto success;
		}

		/* Check against existing mmap mappings. */
		@@ -247,10 +268,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
		/* Ok, looks good - let it rip. */
		if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0)
		goto out;

		set_brk:
		mm->brk = brk;

		success:
		populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0;
		if (downgraded)
		up_read(&mm->mmap_sem);
		else
		up_write(&mm->mmap_sem);
		userfaultfd_unmap_complete(mm, &uf);
		if (populate)
		@@ -258,7 +282,7 @@ set_brk:
		return brk;

		out:
		retval = mm->brk;
		retval = origbrk;
		up_write(&mm->mmap_sem);
		return retval;
		}

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