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Commit 30556047 authored by Peter Xu's avatar Peter Xu Committed by Zhenhua Huang
Browse files

mm/gup: Remove enfornced COW mechanism 



With the more strict (but greatly simplified) page reuse logic in
do_wp_page(), we can safely go back to the world where cow is not
enforced with writes.

This essentially reverts commit 17839856fd58 ("gup: document and work
around 'COW can break either way' issue").  There are some context
differences due to some changes later on around it:

  2170ecfa7688 ("drm/i915: convert get_user_pages() --> pin_user_pages()", 2020-06-03)
  376a34efa4ee ("mm/gup: refactor and de-duplicate gup_fast() code", 2020-06-03)

Some lines moved back and forth with those, but this revert patch should
have striped out and covered all the enforced cow bits anyways.

Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Git-commit: a308c71bf1e6e19cc2e4ced31853ee0fc7cb439a
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


[zhenhuah@codeaurora.org: resolve trivial merge conflict]
Change-Id: I1e72c780c74b324efe6dbeed17ab131657254f16
Signed-off-by: default avatarZhenhua Huang <zhenhuah@codeaurora.org>
parent 6b895253

drivers/gpu/drm/i915/gem/i915_gem_userptr.c

+0 −8
Original line number Diff line number Diff line
@@ -619,14 +619,6 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj) 
				      GFP_KERNEL |

				      __GFP_NORETRY |

				      __GFP_NOWARN);

		/*

		 * Using __get_user_pages_fast() with a read-only

		 * access is questionable. A read-only page may be

		 * COW-broken, and then this might end up giving

		 * the wrong side of the COW..

		 *

		 * We may or may not care.

		 */

		if (pvec) /* defer to worker if malloc fails */

			pinned = __get_user_pages_fast(obj->userptr.ptr,

						       num_pages,

mm/gup.c

+6 −18
Original line number Diff line number Diff line
@@ -161,12 +161,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address, 
}



/*

 * FOLL_FORCE or a forced COW break can write even to unwritable pte's,

 * but only after we've gone through a COW cycle and they are dirty.

 * FOLL_FORCE can write to even unwritable pte's, but only

 * after we've gone through a COW cycle and they are dirty.

 */

static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)

{

	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));

	return pte_write(pte) ||

	    ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));

}



/*
@@ -832,18 +833,12 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm, 
				goto out;

			}

			if (is_vm_hugetlb_page(vma)) {

				if (should_force_cow_break(vma, foll_flags))

					foll_flags |= FOLL_WRITE;

				i = follow_hugetlb_page(mm, vma, pages, vmas,

						&start, &nr_pages, i,

						foll_flags, nonblocking);

						gup_flags, nonblocking);

				continue;

			}

		}



		if (should_force_cow_break(vma, foll_flags))

			foll_flags |= FOLL_WRITE;



retry:

		/*

		 * If we have a pending SIGKILL, don't keep faulting pages and
@@ -2440,17 +2435,10 @@ int get_user_pages_fast(unsigned long start, int nr_pages, 
	if (unlikely(!access_ok((void __user *)start, len)))

		return -EFAULT;



	/*

	 * The FAST_GUP case requires FOLL_WRITE even for pure reads,

	 * because get_user_pages() may need to cause an early COW in

	 * order to avoid confusing the normal COW routines. So only

	 * targets that are already writable are safe to do by just

	 * looking at the page tables.

	 */

	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&

	    gup_fast_permitted(start, end)) {

		local_irq_disable();

		gup_pgd_range(addr, end, gup_flags | FOLL_WRITE, pages, &nr);

		gup_pgd_range(addr, end, gup_flags, pages, &nr);

		local_irq_enable();

		ret = nr;

	}

mm/huge_memory.c

+4 −3
Original line number Diff line number Diff line
@@ -1454,12 +1454,13 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd) 
}



/*

 * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,

 * but only after we've gone through a COW cycle and they are dirty.

 * FOLL_FORCE can write to even unwritable pmd's, but only

 * after we've gone through a COW cycle and they are dirty.

 */

static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)

{

	return pmd_write(pmd) || ((flags & FOLL_COW) && pmd_dirty(pmd));

	return pmd_write(pmd) ||

	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));

}



struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,