| Message ID | 20230602225747.103865-2-mike.kravetz@oracle.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | RESEND fix page_cache_next/prev_miss off by one error | expand |
On Fri, 2 Jun 2023 15:57:47 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote: > Ackerley Tng reported an issue with hugetlbfs fallocate here[1]. The > issue showed up after the conversion of hugetlb page cache lookup code > to use page_cache_next_miss. So I'm assuming Fixes: d0ce0e47b323 ("mm/hugetlb: convert hugetlb fault paths to use alloc_hugetlb_folio()") ? > Code in hugetlb fallocate, userfaultfd > and GUP is now using page_cache_next_miss to determine if a page is > present the page cache. The following statement is used. > > present = page_cache_next_miss(mapping, index, 1) != index; > > There are two issues with page_cache_next_miss when used in this way. > 1) If the passed value for index is equal to the 'wrap-around' value, > the same index will always be returned. This wrap-around value is 0, > so 0 will be returned even if page is present at index 0. > 2) If there is no gap in the range passed, the last index in the range > will be returned. When passed a range of 1 as above, the passed > index value will be returned even if the page is present. > The end result is the statement above will NEVER indicate a page is > present in the cache, even if it is. > > As noted by Ackerley in [1], users can see this by hugetlb fallocate > incorrectly returning EEXIST if pages are already present in the file. > In addition, hugetlb pages will not be included in core dumps if they > need to be brought in via GUP. userfaultfd UFFDIO_COPY also uses this > code and will not notice pages already present in the cache. It may try > to allocate a new page and potentially return ENOMEM as opposed to > EEXIST. > > Both page_cache_next_miss and page_cache_prev_miss have similar issues. > Fix by: > - Check for index equal to 'wrap-around' value and do not exit early. > - If no gap is found in range, return index outside range. > - Update function description to say 'wrap-around' value could be > returned if passed as index. > > [1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@google.com/ >
On 06/02/23 17:59, Andrew Morton wrote: > On Fri, 2 Jun 2023 15:57:47 -0700 Mike Kravetz <mike.kravetz@oracle.com> wrote: > > > Ackerley Tng reported an issue with hugetlbfs fallocate here[1]. The > > issue showed up after the conversion of hugetlb page cache lookup code > > to use page_cache_next_miss. > > So I'm assuming > > Fixes: d0ce0e47b323 ("mm/hugetlb: convert hugetlb fault paths to use alloc_hugetlb_folio()") > Yes, that would be preferred. I originally had Fixes: 0d3f92966629 ("page cache: Convert hole search to XArray") where page_cache_next/prev_miss were introduced. But, there is no issue until the new hugetlb usage.
On 06/05/23 17:26, Ackerley Tng wrote: > Mike Kravetz <mike.kravetz@oracle.com> writes: > > This doesn't seem to work as expected: > > Here's a test I did > > /* Modified so I can pass in an xarray for this test */ > static unsigned long page_cache_next_miss(struct xarray *xa, unsigned long > index, > unsigned long max_scan) > { > XA_STATE(xas, xa, index); > > while (max_scan--) { > void *entry = xas_next(&xas); > if (!entry || xa_is_value(entry)) > return xas.xa_index; > if (xas.xa_index == 0 && index != 0) > return xas.xa_index; > } > > return xas.xa_index + 1; > } > > static noinline void check_find_5(void) > { > struct xarray xa; > unsigned long max_scan; > void *ptr = malloc(10); > > xa_init(&xa); > xa_store_range(&xa, 3, 5, ptr, GFP_KERNEL); > > max_scan = 3; > printk("page_cache_next_miss(xa, %d, %ld): %ld\n", 4, max_scan, > page_cache_next_miss(&xa, 4, max_scan)); > > } > > The above gave me: page_cache_next_miss(xa, 4, 3): 7 > > But I was expecting a return value of 6. > > I investigated a little, and it seems like entry at index 6 if we start > iterating before 6 is 0xe, and xa_is_internal(entry) returns true. > > Not yet familiar with the internals of xarrays, not sure what the fix > should be. I am NOT an expert with xarray. However, the documentation says: "Calling xa_store_range() stores the same entry in a range of indices. If you do this, some of the other operations will behave in a slightly odd way. For example, marking the entry at one index may result in the entry being marked at some, but not all of the other indices. Storing into one index may result in the entry retrieved by some, but not all of the other indices changing." This may be why your test is not functioning as expected? I modified your check_find_5() routine as follows (within lib/test_xarray.c): static noinline void check_find_5(struct xarray *xa, bool mult) { unsigned long max_scan; void *p = &max_scan; XA_BUG_ON(xa, !xa_empty(xa)); if (mult) { xa_store(xa, 3, p, GFP_KERNEL); xa_store(xa, 4, p, GFP_KERNEL); xa_store(xa, 5, p, GFP_KERNEL); } else { xa_store_range(xa, 3, 5, p, GFP_KERNEL); } max_scan = 3; if (mult) printk("---> multiple stores\n"); else printk("---> range store\n"); printk("page_cache_next_miss(xa, %d, %ld): %ld\n", 4, max_scan, __page_cache_next_miss(xa, 4, max_scan)); if (mult) { xa_store(xa, 3, NULL, GFP_KERNEL); xa_store(xa, 4, NULL, GFP_KERNEL); xa_store(xa, 5, NULL, GFP_KERNEL); } else { xa_store_range(xa, 3, 5, NULL, GFP_KERNEL); } xa_destroy(xa); } This results in: [ 149.998676] ---> multiple stores [ 149.999391] page_cache_next_miss(xa, 4, 3): 6 [ 150.003342] ---> range store [ 150.007002] page_cache_next_miss(xa, 4, 3): 7 I am fairly confident the page cache code will make individual xa_store calls as opposed to xa_store_range.
diff --git a/mm/filemap.c b/mm/filemap.c index 71dc90f64e43..123540c7ba45 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1733,7 +1733,9 @@ bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm, * * Return: The index of the gap if found, otherwise an index outside the * range specified (in which case 'return - index >= max_scan' will be true). - * In the rare case of index wrap-around, 0 will be returned. + * In the rare case of index wrap-around, 0 will be returned. 0 will also + * be returned if index == 0 and there is a gap at the index. We can not + * wrap-around if passed index == 0. */ pgoff_t page_cache_next_miss(struct address_space *mapping, pgoff_t index, unsigned long max_scan) @@ -1743,12 +1745,13 @@ pgoff_t page_cache_next_miss(struct address_space *mapping, while (max_scan--) { void *entry = xas_next(&xas); if (!entry || xa_is_value(entry)) - break; - if (xas.xa_index == 0) - break; + return xas.xa_index; + if (xas.xa_index == 0 && index != 0) + return xas.xa_index; } - return xas.xa_index; + /* No gaps in range and no wrap-around, return index beyond range */ + return xas.xa_index + 1; } EXPORT_SYMBOL(page_cache_next_miss); @@ -1769,7 +1772,9 @@ EXPORT_SYMBOL(page_cache_next_miss); * * Return: The index of the gap if found, otherwise an index outside the * range specified (in which case 'index - return >= max_scan' will be true). - * In the rare case of wrap-around, ULONG_MAX will be returned. + * In the rare case of wrap-around, ULONG_MAX will be returned. ULONG_MAX + * will also be returned if index == ULONG_MAX and there is a gap at the + * index. We can not wrap-around if passed index == ULONG_MAX. */ pgoff_t page_cache_prev_miss(struct address_space *mapping, pgoff_t index, unsigned long max_scan) @@ -1779,12 +1784,13 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping, while (max_scan--) { void *entry = xas_prev(&xas); if (!entry || xa_is_value(entry)) - break; - if (xas.xa_index == ULONG_MAX) - break; + return xas.xa_index; + if (xas.xa_index == ULONG_MAX && index != ULONG_MAX) + return xas.xa_index; } - return xas.xa_index; + /* No gaps in range and no wrap-around, return index beyond range */ + return xas.xa_index - 1; } EXPORT_SYMBOL(page_cache_prev_miss);
Ackerley Tng reported an issue with hugetlbfs fallocate here[1]. The issue showed up after the conversion of hugetlb page cache lookup code to use page_cache_next_miss. Code in hugetlb fallocate, userfaultfd and GUP is now using page_cache_next_miss to determine if a page is present the page cache. The following statement is used. present = page_cache_next_miss(mapping, index, 1) != index; There are two issues with page_cache_next_miss when used in this way. 1) If the passed value for index is equal to the 'wrap-around' value, the same index will always be returned. This wrap-around value is 0, so 0 will be returned even if page is present at index 0. 2) If there is no gap in the range passed, the last index in the range will be returned. When passed a range of 1 as above, the passed index value will be returned even if the page is present. The end result is the statement above will NEVER indicate a page is present in the cache, even if it is. As noted by Ackerley in [1], users can see this by hugetlb fallocate incorrectly returning EEXIST if pages are already present in the file. In addition, hugetlb pages will not be included in core dumps if they need to be brought in via GUP. userfaultfd UFFDIO_COPY also uses this code and will not notice pages already present in the cache. It may try to allocate a new page and potentially return ENOMEM as opposed to EEXIST. Both page_cache_next_miss and page_cache_prev_miss have similar issues. Fix by: - Check for index equal to 'wrap-around' value and do not exit early. - If no gap is found in range, return index outside range. - Update function description to say 'wrap-around' value could be returned if passed as index. [1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@google.com/ Reported-by: Ackerley Tng <ackerleytng@google.com> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com> --- mm/filemap.c | 26 ++++++++++++++++---------- 1 file changed, 16 insertions(+), 10 deletions(-)