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Revert "gup: document and work around "COW can break either way" issue"

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This reverts commit 9bbd42e79720122334226afad9ddcac1c3e6d373, which
was commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.  The
backport was incorrect and incomplete:

* It forced the write flag on in the generic __get_user_pages_fast(),
  whereas only get_user_pages_fast() was supposed to do that.
* It only fixed the generic RCU-based implementation used by arm,
  arm64, and powerpc.  Before Linux 4.13, several other architectures
  had their own implementations: mips, s390, sparc, sh, and x86.

This will be followed by a (hopefully) correct backport.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ben Hutchings 2022-01-24 14:38:28 +01:00 committed by Greg Kroah-Hartman
parent 31f961674d
commit 6fbb838388
2 changed files with 12 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

48
mm/gup.c
View File

@ -61,22 +61,13 @@ 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));
}
/*
* A (separate) COW fault might break the page the other way and
* get_user_pages() would return the page from what is now the wrong
* VM. So we need to force a COW break at GUP time even for reads.
*/
static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
{
return is_cow_mapping(vma->vm_flags) && (flags & FOLL_GET);
return pte_write(pte) ||
((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}
static struct page *follow_page_pte(struct vm_area_struct *vma,
@ -586,18 +577,12 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
if (!vma || check_vma_flags(vma, gup_flags))
return i ? : -EFAULT;
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);
gup_flags);
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
@ -1518,10 +1503,6 @@ static int gup_pud_range(pgd_t pgd, unsigned long addr, unsigned long end,
/*
* Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
* the regular GUP. It will only return non-negative values.
*
* Careful, careful! COW breaking can go either way, so a non-write
* access can get ambiguous page results. If you call this function without
* 'write' set, you'd better be sure that you're ok with that ambiguity.
*/
int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
struct page **pages)
@ -1551,12 +1532,6 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
*
* We do not adopt an rcu_read_lock(.) here as we also want to
* block IPIs that come from THPs splitting.
*
* NOTE! We allow read-only gup_fast() here, but you'd better be
* careful about possible COW pages. You'll get _a_ COW page, but
* not necessarily the one you intended to get depending on what
* COW event happens after this. COW may break the page copy in a
* random direction.
*/
local_irq_save(flags);
@ -1567,22 +1542,15 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
next = pgd_addr_end(addr, end);
if (pgd_none(pgd))
break;
/*
* 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 (unlikely(pgd_huge(pgd))) {
if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
pages, &nr))
break;
} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
PGDIR_SHIFT, next, 1, pages, &nr))
PGDIR_SHIFT, next, write, pages, &nr))
break;
} else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
break;
} while (pgdp++, addr = next, addr != end);
local_irq_restore(flags);

7
mm/huge_memory.c
View File

@ -1135,12 +1135,13 @@ out_unlock:
}
/*
* 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,