| Message ID | 20170526195932.32178-1-ross.zwisler@linux.intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri 26-05-17 13:59:32, Ross Zwisler wrote: > This commit, which has not yet made it upstream but is in the -mm tree: > > dax: Fix race between colliding PMD & PTE entries > > fixed a pair of race conditions where racing DAX PTE and PMD faults could > corrupt page tables. This fix had two shortcomings which are addressed by > this patch: > > 1) In the PTE fault handler we only checked for a collision using > pmd_devmap(). The pmd_devmap() check will trigger when we have raced with > a PMD that has real DAX storage, but to account for the case where we > collide with a huge zero page entry we also need to check for > pmd_trans_huge(). > > 2) In the PMD fault handler we only continued with the fault if no PMD at > all was present (pmd_none()). This is the case when we are faulting in a > PMD for the first time, but there are two other cases to consider. The > first is that we are servicing a write fault over a PMD huge zero page, > which we detect with pmd_trans_huge(). The second is that we are servicing > a write fault over a DAX PMD with real storage, which we address with > pmd_devmap(). > > Fix both of these, and instead of manually triggering a fallback in the PMD > collision case instead be consistent with the other collision detection > code in the fault handlers and just retry. > > Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> > Cc: stable@vger.kernel.org Ugh, right. I forgot zero page in page tables will not pass the devmap check... You can add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > > For both the -mm tree and for stable, feel free to squash this with the > original commit if you think that is appropriate. > > This has passed targeted testing and an xfstests run. > --- > fs/dax.c | 11 +++++++---- > 1 file changed, 7 insertions(+), 4 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index fc62f36..2a6889b 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -1160,7 +1160,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, > * the PTE we need to set up. If so just return and the fault will be > * retried. > */ > - if (pmd_devmap(*vmf->pmd)) { > + if (pmd_trans_huge(*vmf->pmd) || pmd_devmap(*vmf->pmd)) { > vmf_ret = VM_FAULT_NOPAGE; > goto unlock_entry; > } > @@ -1411,11 +1411,14 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, > /* > * It is possible, particularly with mixed reads & writes to private > * mappings, that we have raced with a PTE fault that overlaps with > - * the PMD we need to set up. If so we just fall back to a PTE fault > - * ourselves. > + * the PMD we need to set up. If so just return and the fault will be > + * retried. > */ > - if (!pmd_none(*vmf->pmd)) > + if (!pmd_none(*vmf->pmd) && !pmd_trans_huge(*vmf->pmd) && > + !pmd_devmap(*vmf->pmd)) { > + result = 0; > goto unlock_entry; > + } > > /* > * Note that we don't use iomap_apply here. We aren't doing I/O, only > -- > 2.9.4 >
diff --git a/fs/dax.c b/fs/dax.c index fc62f36..2a6889b 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -1160,7 +1160,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, * the PTE we need to set up. If so just return and the fault will be * retried. */ - if (pmd_devmap(*vmf->pmd)) { + if (pmd_trans_huge(*vmf->pmd) || pmd_devmap(*vmf->pmd)) { vmf_ret = VM_FAULT_NOPAGE; goto unlock_entry; } @@ -1411,11 +1411,14 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, /* * It is possible, particularly with mixed reads & writes to private * mappings, that we have raced with a PTE fault that overlaps with - * the PMD we need to set up. If so we just fall back to a PTE fault - * ourselves. + * the PMD we need to set up. If so just return and the fault will be + * retried. */ - if (!pmd_none(*vmf->pmd)) + if (!pmd_none(*vmf->pmd) && !pmd_trans_huge(*vmf->pmd) && + !pmd_devmap(*vmf->pmd)) { + result = 0; goto unlock_entry; + } /* * Note that we don't use iomap_apply here. We aren't doing I/O, only
This commit, which has not yet made it upstream but is in the -mm tree: dax: Fix race between colliding PMD & PTE entries fixed a pair of race conditions where racing DAX PTE and PMD faults could corrupt page tables. This fix had two shortcomings which are addressed by this patch: 1) In the PTE fault handler we only checked for a collision using pmd_devmap(). The pmd_devmap() check will trigger when we have raced with a PMD that has real DAX storage, but to account for the case where we collide with a huge zero page entry we also need to check for pmd_trans_huge(). 2) In the PMD fault handler we only continued with the fault if no PMD at all was present (pmd_none()). This is the case when we are faulting in a PMD for the first time, but there are two other cases to consider. The first is that we are servicing a write fault over a PMD huge zero page, which we detect with pmd_trans_huge(). The second is that we are servicing a write fault over a DAX PMD with real storage, which we address with pmd_devmap(). Fix both of these, and instead of manually triggering a fallback in the PMD collision case instead be consistent with the other collision detection code in the fault handlers and just retry. Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Cc: stable@vger.kernel.org --- For both the -mm tree and for stable, feel free to squash this with the original commit if you think that is appropriate. This has passed targeted testing and an xfstests run. --- fs/dax.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)