On Tue, Sep 22, 2015 at 2:17 PM, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > On Tue, Sep 22, 2015 at 01:51:04PM -0700, Dan Williams wrote: >> [ adding Andrew ] >> >> On Tue, Sep 22, 2015 at 12:36 PM, Ross Zwisler >> <ross.zwisler@xxxxxxxxxxxxxxx> wrote: >> > The following commit: >> > >> > commit 46c043ede471 ("mm: take i_mmap_lock in unmap_mapping_range() for >> > DAX") >> > >> > moved some code in __dax_pmd_fault() that was responsible for zeroing >> > newly allocated PMD pages. The new location didn't properly set up >> > 'kaddr', though, so when run this code resulted in a NULL pointer BUG. >> > >> > Fix this by getting the correct 'kaddr' via bdev_direct_access(). >> > >> > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> >> > Reported-by: Dan Williams <dan.j.williams@xxxxxxxxx> >> >> Taking into account the comment below, >> >> Reviewed-by: Dan Williams <dan.j.williams@xxxxxxxxx> >> >> > --- >> > fs/dax.c | 13 ++++++++++++- >> > 1 file changed, 12 insertions(+), 1 deletion(-) >> > >> > diff --git a/fs/dax.c b/fs/dax.c >> > index 7ae6df7..bcfb14b 100644 >> > --- a/fs/dax.c >> > +++ b/fs/dax.c >> > @@ -569,8 +569,20 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address, >> > if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) >> > goto fallback; >> > >> > + sector = bh.b_blocknr << (blkbits - 9); >> > + >> > if (buffer_unwritten(&bh) || buffer_new(&bh)) { >> > int i; >> > + >> > + length = bdev_direct_access(bh.b_bdev, sector, &kaddr, &pfn, >> > + bh.b_size); >> > + if (length < 0) { >> > + result = VM_FAULT_SIGBUS; >> > + goto out; >> > + } >> > + if ((length < PMD_SIZE) || (pfn & PG_PMD_COLOUR)) >> > + goto fallback; >> > + >> >> Hmm, we don't need the PG_PMD_COLOUR check since we aren't using the >> pfn in this path, right? > > I think we care, because we'll end up bailing anyway at the later > PG_PMD_COLOUR check before we actually insert the pfn via > vmf_insert_pfn_pmd(). If we don't check the alignment we'll do 2 MiB worth of > zeroing to the media, then later fall back to PTE faults. Ok, good point. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>