| Message ID | 20170510172700.18991-1-ross.zwisler@linux.intel.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 876f29460cbd |
| Headers | show |
On Wed 10-05-17 11:27:00, Ross Zwisler wrote: > This is based on a patch from Jan Kara that fixed the equivalent race in > the DAX PTE fault path. > > Currently DAX PMD read fault can race with write(2) in the following way: > > CPU1 - write(2) CPU2 - read fault > dax_iomap_pmd_fault() > ->iomap_begin() - sees hole > > dax_iomap_rw() > iomap_apply() > ->iomap_begin - allocates blocks > dax_iomap_actor() > invalidate_inode_pages2_range() > - there's nothing to invalidate > > grab_mapping_entry() > - we add huge zero page to the radix tree > and map it to page tables > > The result is that hole page is mapped into page tables (and thus zeros > are seen in mmap) while file has data written in that place. > > Fix the problem by locking exception entry before mapping blocks for the > fault. That way we are sure invalidate_inode_pages2_range() call for > racing write will either block on entry lock waiting for the fault to > finish (and unmap stale page tables after that) or read fault will see > already allocated blocks by write(2). > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > Fixes: 9f141d6ef6258a3a37a045842d9ba7e68f368956 > CC: stable@vger.kernel.org > --- > > Jan, I just realized that we need an equivalent fix in the PMD path. Let's > keep this with the rest of your series so they get applied together, > applied to stable together, etc. > > This applies cleanly to the current linux/master (56868a460b83) + the four > patches from Jan's series. I've run it through xfstests and some targeted > testing for the PMD path. Ah, right. Thanks for fixing it up. The patch looks good. You can add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > fs/dax.c | 28 ++++++++++++++-------------- > 1 file changed, 14 insertions(+), 14 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 32f020c..93ae872 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -1388,6 +1388,16 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > goto fallback; > > /* > + * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX > + * PMD or a HZP entry. If it can't (because a 4k page is already in > + * the tree, for instance), it will return -EEXIST and we just fall > + * back to 4k entries. > + */ > + entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD); > + if (IS_ERR(entry)) > + goto fallback; > + > + /* > * Note that we don't use iomap_apply here. We aren't doing I/O, only > * setting up a mapping, so really we're using iomap_begin() as a way > * to look up our filesystem block. > @@ -1395,21 +1405,11 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > pos = (loff_t)pgoff << PAGE_SHIFT; > error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap); > if (error) > - goto fallback; > + goto unlock_entry; > > if (iomap.offset + iomap.length < pos + PMD_SIZE) > goto finish_iomap; > > - /* > - * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX > - * PMD or a HZP entry. If it can't (because a 4k page is already in > - * the tree, for instance), it will return -EEXIST and we just fall > - * back to 4k entries. > - */ > - entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD); > - if (IS_ERR(entry)) > - goto finish_iomap; > - > switch (iomap.type) { > case IOMAP_MAPPED: > result = dax_pmd_insert_mapping(vmf, &iomap, pos, &entry); > @@ -1417,7 +1417,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > case IOMAP_UNWRITTEN: > case IOMAP_HOLE: > if (WARN_ON_ONCE(write)) > - goto unlock_entry; > + break; > result = dax_pmd_load_hole(vmf, &iomap, &entry); > break; > default: > @@ -1425,8 +1425,6 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > break; > } > > - unlock_entry: > - put_locked_mapping_entry(mapping, pgoff, entry); > finish_iomap: > if (ops->iomap_end) { > int copied = PMD_SIZE; > @@ -1442,6 +1440,8 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > ops->iomap_end(inode, pos, PMD_SIZE, copied, iomap_flags, > &iomap); > } > + unlock_entry: > + put_locked_mapping_entry(mapping, pgoff, entry); > fallback: > if (result == VM_FAULT_FALLBACK) { > split_huge_pmd(vma, vmf->pmd, vmf->address); > -- > 2.9.3 >
diff --git a/fs/dax.c b/fs/dax.c index 32f020c..93ae872 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -1388,6 +1388,16 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, goto fallback; /* + * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX + * PMD or a HZP entry. If it can't (because a 4k page is already in + * the tree, for instance), it will return -EEXIST and we just fall + * back to 4k entries. + */ + entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD); + if (IS_ERR(entry)) + goto fallback; + + /* * Note that we don't use iomap_apply here. We aren't doing I/O, only * setting up a mapping, so really we're using iomap_begin() as a way * to look up our filesystem block. @@ -1395,21 +1405,11 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, pos = (loff_t)pgoff << PAGE_SHIFT; error = ops->iomap_begin(inode, pos, PMD_SIZE, iomap_flags, &iomap); if (error) - goto fallback; + goto unlock_entry; if (iomap.offset + iomap.length < pos + PMD_SIZE) goto finish_iomap; - /* - * grab_mapping_entry() will make sure we get a 2M empty entry, a DAX - * PMD or a HZP entry. If it can't (because a 4k page is already in - * the tree, for instance), it will return -EEXIST and we just fall - * back to 4k entries. - */ - entry = grab_mapping_entry(mapping, pgoff, RADIX_DAX_PMD); - if (IS_ERR(entry)) - goto finish_iomap; - switch (iomap.type) { case IOMAP_MAPPED: result = dax_pmd_insert_mapping(vmf, &iomap, pos, &entry); @@ -1417,7 +1417,7 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, case IOMAP_UNWRITTEN: case IOMAP_HOLE: if (WARN_ON_ONCE(write)) - goto unlock_entry; + break; result = dax_pmd_load_hole(vmf, &iomap, &entry); break; default: @@ -1425,8 +1425,6 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, break; } - unlock_entry: - put_locked_mapping_entry(mapping, pgoff, entry); finish_iomap: if (ops->iomap_end) { int copied = PMD_SIZE; @@ -1442,6 +1440,8 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, ops->iomap_end(inode, pos, PMD_SIZE, copied, iomap_flags, &iomap); } + unlock_entry: + put_locked_mapping_entry(mapping, pgoff, entry); fallback: if (result == VM_FAULT_FALLBACK) { split_huge_pmd(vma, vmf->pmd, vmf->address);
This is based on a patch from Jan Kara that fixed the equivalent race in the DAX PTE fault path. Currently DAX PMD read fault can race with write(2) in the following way: CPU1 - write(2) CPU2 - read fault dax_iomap_pmd_fault() ->iomap_begin() - sees hole dax_iomap_rw() iomap_apply() ->iomap_begin - allocates blocks dax_iomap_actor() invalidate_inode_pages2_range() - there's nothing to invalidate grab_mapping_entry() - we add huge zero page to the radix tree and map it to page tables The result is that hole page is mapped into page tables (and thus zeros are seen in mmap) while file has data written in that place. Fix the problem by locking exception entry before mapping blocks for the fault. That way we are sure invalidate_inode_pages2_range() call for racing write will either block on entry lock waiting for the fault to finish (and unmap stale page tables after that) or read fault will see already allocated blocks by write(2). Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Fixes: 9f141d6ef6258a3a37a045842d9ba7e68f368956 CC: stable@vger.kernel.org --- Jan, I just realized that we need an equivalent fix in the PMD path. Let's keep this with the rest of your series so they get applied together, applied to stable together, etc. This applies cleanly to the current linux/master (56868a460b83) + the four patches from Jan's series. I've run it through xfstests and some targeted testing for the PMD path. --- fs/dax.c | 28 ++++++++++++++-------------- 1 file changed, 14 insertions(+), 14 deletions(-)