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Date: Thu, 7 Jan 2016 08:46:06 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: Hillf Danton <hillf.zj@...baba-inc.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: "'Hugh Dickins'" <hughd@...gle.com>,
"'Naoya Horiguchi'" <n-horiguchi@...jp.nec.com>,
"'Davidlohr Bueso'" <dave@...olabs.net>,
"'Dave Hansen'" <dave.hansen@...ux.intel.com>,
"'Andrew Morton'" <akpm@...ux-foundation.org>,
"'Michel Lespinasse'" <walken@...gle.com>
Subject: Re: [PATCH] mm/hugetlbfs: Unmap pages if page fault raced with hole
punch
On 01/07/2016 12:06 AM, Hillf Danton wrote:
>>
>> Page faults can race with fallocate hole punch. If a page fault happens
>> between the unmap and remove operations, the page is not removed and
>> remains within the hole. This is not the desired behavior. The race
>> is difficult to detect in user level code as even in the non-race
>> case, a page within the hole could be faulted back in before fallocate
>> returns. If userfaultfd is expanded to support hugetlbfs in the future,
>> this race will be easier to observe.
>>
>> If this race is detected and a page is mapped, the remove operation
>> (remove_inode_hugepages) will unmap the page before removing. The unmap
>> within remove_inode_hugepages occurs with the hugetlb_fault_mutex held
>> so that no other faults will be processed until the page is removed.
>>
>> The (unmodified) routine hugetlb_vmdelete_list was moved ahead of
>> remove_inode_hugepages to satisfy the new reference.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>> ---
>> fs/hugetlbfs/inode.c | 139 +++++++++++++++++++++++++++------------------------
>> 1 file changed, 73 insertions(+), 66 deletions(-)
>>
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index 0444760..0871d70 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -324,11 +324,46 @@ static void remove_huge_page(struct page *page)
>> delete_from_page_cache(page);
>> }
>>
>> +static inline void
>> +hugetlb_vmdelete_list(struct rb_root *root, pgoff_t start, pgoff_t end)
>> +{
>> + struct vm_area_struct *vma;
>> +
>> + /*
>> + * end == 0 indicates that the entire range after
>> + * start should be unmapped.
>> + */
>> + vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
>
> [1] perhaps end can be reused.
>
>> + unsigned long v_offset;
>> +
>> + /*
>> + * Can the expression below overflow on 32-bit arches?
>> + * No, because the interval tree returns us only those vmas
>> + * which overlap the truncated area starting at pgoff,
>> + * and no vma on a 32-bit arch can span beyond the 4GB.
>> + */
>> + if (vma->vm_pgoff < start)
>> + v_offset = (start - vma->vm_pgoff) << PAGE_SHIFT;
>> + else
>> + v_offset = 0;
>> +
>> + if (end) {
>> + end = ((end - start) << PAGE_SHIFT) +
>> + vma->vm_start + v_offset;
>
> [2] end is input to be pgoff_t, but changed to be the type of v_offset.
> Further we cannot handle the case that end is input to be zero.
> See the diff below please.
Thanks Hillf.
This bug is part of the existing code. I did not modify current
hugetlb_vmdelete_list code, just moved it as part of this patch.
Therefore, I will create a separate patch to fix this issue.
--
Mike Kravetz
>
>> + if (end > vma->vm_end)
>> + end = vma->vm_end;
>> + } else
>> + end = vma->vm_end;
>> +
>> + unmap_hugepage_range(vma, vma->vm_start + v_offset, end, NULL);
>> + }
>> +}
>> +
>>
>> /*
>> * remove_inode_hugepages handles two distinct cases: truncation and hole
>> * punch. There are subtle differences in operation for each case.
>> -
>> + *
>> * truncation is indicated by end of range being LLONG_MAX
>> * In this case, we first scan the range and release found pages.
>> * After releasing pages, hugetlb_unreserve_pages cleans up region/reserv
>> @@ -379,6 +414,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>>
>> for (i = 0; i < pagevec_count(&pvec); ++i) {
>> struct page *page = pvec.pages[i];
>> + bool rsv_on_error;
>> u32 hash;
>>
>> /*
>> @@ -395,37 +431,43 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
>> mapping, next, 0);
>> mutex_lock(&hugetlb_fault_mutex_table[hash]);
>>
>> - lock_page(page);
>> - if (likely(!page_mapped(page))) {
>> - bool rsv_on_error = !PagePrivate(page);
>> - /*
>> - * We must free the huge page and remove
>> - * from page cache (remove_huge_page) BEFORE
>> - * removing the region/reserve map
>> - * (hugetlb_unreserve_pages). In rare out
>> - * of memory conditions, removal of the
>> - * region/reserve map could fail. Before
>> - * free'ing the page, note PagePrivate which
>> - * is used in case of error.
>> - */
>> - remove_huge_page(page);
>> - freed++;
>> - if (!truncate_op) {
>> - if (unlikely(hugetlb_unreserve_pages(
>> - inode, next,
>> - next + 1, 1)))
>> - hugetlb_fix_reserve_counts(
>> - inode, rsv_on_error);
>> - }
>> - } else {
>> - /*
>> - * If page is mapped, it was faulted in after
>> - * being unmapped. It indicates a race between
>> - * hole punch and page fault. Do nothing in
>> - * this case. Getting here in a truncate
>> - * operation is a bug.
>> - */
>> + /*
>> + * If page is mapped, it was faulted in after being
>> + * unmapped in caller. Unmap (again) now after taking
>> + * the fault mutex. The mutex will prevent faults
>> + * until we finish removing the page.
>> + *
>> + * This race can only happen in the hole punch case.
>> + * Getting here in a truncate operation is a bug.
>> + */
>> + if (unlikely(page_mapped(page))) {
>> BUG_ON(truncate_op);
>> +
>> + i_mmap_lock_write(mapping);
>> + hugetlb_vmdelete_list(&mapping->i_mmap,
>> + next * pages_per_huge_page(h),
>> + (next + 1) * pages_per_huge_page(h));
>> + i_mmap_unlock_write(mapping);
>> + }
>> +
>> + lock_page(page);
>> + /*
>> + * We must free the huge page and remove from page
>> + * cache (remove_huge_page) BEFORE removing the
>> + * region/reserve map (hugetlb_unreserve_pages). In
>> + * rare out of memory conditions, removal of the
>> + * region/reserve map could fail. Before free'ing
>> + * the page, note PagePrivate which is used in case
>> + * of error.
>> + */
>> + rsv_on_error = !PagePrivate(page);
>> + remove_huge_page(page);
>> + freed++;
>> + if (!truncate_op) {
>> + if (unlikely(hugetlb_unreserve_pages(inode,
>> + next, next + 1, 1)))
>> + hugetlb_fix_reserve_counts(inode,
>> + rsv_on_error);
>> }
>>
>> unlock_page(page);
>> @@ -452,41 +494,6 @@ static void hugetlbfs_evict_inode(struct inode *inode)
>> clear_inode(inode);
>> }
>>
>> -static inline void
>> -hugetlb_vmdelete_list(struct rb_root *root, pgoff_t start, pgoff_t end)
>> -{
>> - struct vm_area_struct *vma;
>> -
>> - /*
>> - * end == 0 indicates that the entire range after
>> - * start should be unmapped.
>> - */
>> - vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
>> - unsigned long v_offset;
>> -
>> - /*
>> - * Can the expression below overflow on 32-bit arches?
>> - * No, because the interval tree returns us only those vmas
>> - * which overlap the truncated area starting at pgoff,
>> - * and no vma on a 32-bit arch can span beyond the 4GB.
>> - */
>> - if (vma->vm_pgoff < start)
>> - v_offset = (start - vma->vm_pgoff) << PAGE_SHIFT;
>> - else
>> - v_offset = 0;
>> -
>> - if (end) {
>> - end = ((end - start) << PAGE_SHIFT) +
>> - vma->vm_start + v_offset;
>> - if (end > vma->vm_end)
>> - end = vma->vm_end;
>> - } else
>> - end = vma->vm_end;
>> -
>> - unmap_hugepage_range(vma, vma->vm_start + v_offset, end, NULL);
>> - }
>> -}
>> -
>> static int hugetlb_vmtruncate(struct inode *inode, loff_t offset)
>> {
>> pgoff_t pgoff;
>> --
>> 2.4.3
>>
>
> --- a/fs/hugetlbfs/inode.c Thu Jan 7 15:04:35 2016
> +++ b/fs/hugetlbfs/inode.c Thu Jan 7 15:31:03 2016
> @@ -461,8 +461,11 @@ hugetlb_vmdelete_list(struct rb_root *ro
> * end == 0 indicates that the entire range after
> * start should be unmapped.
> */
> - vma_interval_tree_foreach(vma, root, start, end ? end : ULONG_MAX) {
> + if (!end)
> + end = ULONG_MAX;
> + vma_interval_tree_foreach(vma, root, start, end) {
> unsigned long v_offset;
> + unsigned long v_end;
>
> /*
> * Can the expression below overflow on 32-bit arches?
> @@ -475,15 +478,12 @@ hugetlb_vmdelete_list(struct rb_root *ro
> else
> v_offset = 0;
>
> - if (end) {
> - end = ((end - start) << PAGE_SHIFT) +
> + v_end = ((end - start) << PAGE_SHIFT) +
> vma->vm_start + v_offset;
> - if (end > vma->vm_end)
> - end = vma->vm_end;
> - } else
> - end = vma->vm_end;
> + if (v_end > vma->vm_end)
> + v_end = vma->vm_end;
>
> - unmap_hugepage_range(vma, vma->vm_start + v_offset, end, NULL);
> + unmap_hugepage_range(vma, vma->vm_start + v_offset, v_end, NULL);
> }
> }
>
> --
>
--
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