| Message ID | 20220603205741.12888-1-axelrasmussen@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages | expand |
On Fri, 3 Jun 2022, Axel Rasmussen wrote: > When fallocate() is used on a shmem file, the pages we allocate can end > up with !PageUptodate. > > Since UFFDIO_CONTINUE tries to find the existing page the user wants to > map with SGP_READ, we would fail to find such a page, since > shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it > discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, > as it would do if the page wasn't found in the page cache at all. > > This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find > if a page exists, and doesn't care whether it still needs to be cleared > or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, > except for one critical difference: in the !PageUptodate case, > SGP_NOALLOC will clear the page and then return it. With this change, > UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been > fallocated, but otherwise not modified. > > Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") > Cc: stable@vger.kernel.org > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> Acked-by: Hugh Dickins <hughd@google.com> Yes, that looks like a good improvement, and okay to call it a fix for stable. But note that your original choice of SGP_READ was not wrong: there was no SGP_NOALLOC in v5.14, which arrived in v5.15 (and this is not the first time that it's turned out to be useful since then). Since v5.14-stable reached end-of-life some time ago, marking this patch for stable should not result in any untoward build errors. > --- > mm/userfaultfd.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 4f4892a5f767..c156f7f5b854 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -246,7 +246,7 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm, > struct page *page; > int ret; > > - ret = shmem_getpage(inode, pgoff, &page, SGP_READ); > + ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); > if (ret) > goto out; > if (!page) { > -- > 2.36.1.255.ge46751e96f-goog
Hi, Axel, Sorry to read this late. On Fri, Jun 03, 2022 at 01:57:41PM -0700, Axel Rasmussen wrote: > When fallocate() is used on a shmem file, the pages we allocate can end > up with !PageUptodate. > > Since UFFDIO_CONTINUE tries to find the existing page the user wants to > map with SGP_READ, we would fail to find such a page, since > shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it > discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, > as it would do if the page wasn't found in the page cache at all. > > This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find > if a page exists, and doesn't care whether it still needs to be cleared > or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, > except for one critical difference: in the !PageUptodate case, > SGP_NOALLOC will clear the page and then return it. With this change, > UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been > fallocated, but otherwise not modified. > > Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") > Cc: stable@vger.kernel.org > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> > --- > mm/userfaultfd.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > index 4f4892a5f767..c156f7f5b854 100644 > --- a/mm/userfaultfd.c > +++ b/mm/userfaultfd.c > @@ -246,7 +246,7 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm, > struct page *page; > int ret; > > - ret = shmem_getpage(inode, pgoff, &page, SGP_READ); > + ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); > if (ret) > goto out; > if (!page) { It all looks sane if the page is !uptodate as you described. Though I've a question on what'll happen if the page is actually missing rather than just !PageUptodate(). My reading is previously it'll keep returning 0 on shmem_getpage_gfp() for both cases, but now for the missing page shmem_getpage_gfp() will return -ENOENT instead. This reminded me on whether this will errornously let __mcopy_atomic() go into the special path to copy the page without mmap lock, please see this commit: b6ebaedb4cb1 ("userfaultfd: avoid mmap_sem read recursion in mcopy_atomic", 2015-09-04) Would that be a problem? Or could I read it wrong? This also reminded me that whether we'd better need some protection in the -ENOENT handling in __mcopy_atomic() to be always safe. Thanks,
Ah, thanks for pointing this out Peter, it is definitely something I missed. You're right that elsewhere in mm/userfaultfd.c we use -ENOENT for some special case. I think for mcontinue_atomic_pte, we don't want to change the status quo - if we fail to lookup an existing page, we should just return -EFAULT just like we were doing before. We certainly shouldn't return -ENOENT, as that causes us to take a wrong, unrelated code path a couple of callers up, as you mentioned. I'll send a v2 with this small modification. On Thu, Jun 9, 2022 at 2:44 PM Peter Xu <peterx@redhat.com> wrote: > > Hi, Axel, > > Sorry to read this late. > > On Fri, Jun 03, 2022 at 01:57:41PM -0700, Axel Rasmussen wrote: > > When fallocate() is used on a shmem file, the pages we allocate can end > > up with !PageUptodate. > > > > Since UFFDIO_CONTINUE tries to find the existing page the user wants to > > map with SGP_READ, we would fail to find such a page, since > > shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it > > discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, > > as it would do if the page wasn't found in the page cache at all. > > > > This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find > > if a page exists, and doesn't care whether it still needs to be cleared > > or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, > > except for one critical difference: in the !PageUptodate case, > > SGP_NOALLOC will clear the page and then return it. With this change, > > UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been > > fallocated, but otherwise not modified. > > > > Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") > > Cc: stable@vger.kernel.org > > Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> > > --- > > mm/userfaultfd.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > index 4f4892a5f767..c156f7f5b854 100644 > > --- a/mm/userfaultfd.c > > +++ b/mm/userfaultfd.c > > @@ -246,7 +246,7 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm, > > struct page *page; > > int ret; > > > > - ret = shmem_getpage(inode, pgoff, &page, SGP_READ); > > + ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); > > if (ret) > > goto out; > > if (!page) { > > It all looks sane if the page is !uptodate as you described. Though I've a > question on what'll happen if the page is actually missing rather than just > !PageUptodate(). > > My reading is previously it'll keep returning 0 on shmem_getpage_gfp() for > both cases, but now for the missing page shmem_getpage_gfp() will return > -ENOENT instead. > > This reminded me on whether this will errornously let __mcopy_atomic() go > into the special path to copy the page without mmap lock, please see this > commit: > > b6ebaedb4cb1 ("userfaultfd: avoid mmap_sem read recursion in mcopy_atomic", 2015-09-04) > > Would that be a problem? Or could I read it wrong? > > This also reminded me that whether we'd better need some protection in the > -ENOENT handling in __mcopy_atomic() to be always safe. > > Thanks, > > -- > Peter Xu >
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 4f4892a5f767..c156f7f5b854 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -246,7 +246,7 @@ static int mcontinue_atomic_pte(struct mm_struct *dst_mm, struct page *page; int ret; - ret = shmem_getpage(inode, pgoff, &page, SGP_READ); + ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); if (ret) goto out; if (!page) {
When fallocate() is used on a shmem file, the pages we allocate can end up with !PageUptodate. Since UFFDIO_CONTINUE tries to find the existing page the user wants to map with SGP_READ, we would fail to find such a page, since shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, as it would do if the page wasn't found in the page cache at all. This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find if a page exists, and doesn't care whether it still needs to be cleared or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, except for one critical difference: in the !PageUptodate case, SGP_NOALLOC will clear the page and then return it. With this change, UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been fallocated, but otherwise not modified. Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") Cc: stable@vger.kernel.org Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> --- mm/userfaultfd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)