On Wed 23-03-16 14:50:00, Ross Zwisler wrote: > On Mon, Mar 21, 2016 at 02:22:52PM +0100, Jan Kara wrote: > > Currently the handling of huge pages for DAX is racy. For example the > > following can happen: > > > > CPU0 (THP write fault) CPU1 (normal read fault) > > > > __dax_pmd_fault() __dax_fault() > > get_block(inode, block, &bh, 0) -> not mapped > > get_block(inode, block, &bh, 0) > > -> not mapped > > if (!buffer_mapped(&bh) && write) > > get_block(inode, block, &bh, 1) -> allocates blocks > > truncate_pagecache_range(inode, lstart, lend); > > dax_load_hole(); > > > > This results in data corruption since process on CPU1 won't see changes > > into the file done by CPU0. > > > > The race can happen even if two normal faults race however with THP the > > situation is even worse because the two faults don't operate on the same > > entries in the radix tree and we want to use these entries for > > serialization. So disable THP support in DAX code for now. > > Yep, I agree that we should disable PMD faults until we get the multi-order > radix tree work finished and integrated with this locking. > > I do agree with Dan though that it would be preferable to disable PMD faults > by having CONFIG_FS_DAX_PMD depend on BROKEN. That seems smaller and easier > to switch PMD faults back on for testing. I did it this way because I wasn't sure PMD fault code even compiles and I didn't want to break CONFIG_BROKEN builds. But OK, I'll make the code at least compile and do what you say. Honza > > Signed-off-by: Jan Kara <jack@xxxxxxx> > > --- > > fs/dax.c | 2 +- > > include/linux/dax.h | 2 +- > > 2 files changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/fs/dax.c b/fs/dax.c > > index 0329ec0bee2e..444e9dd079ca 100644 > > --- a/fs/dax.c > > +++ b/fs/dax.c > > @@ -719,7 +719,7 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf, > > } > > EXPORT_SYMBOL_GPL(dax_fault); > > > > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE > > +#if 0 > > /* > > * The 'colour' (ie low bits) within a PMD of a page offset. This comes up > > * more often than one might expect in the below function. > > diff --git a/include/linux/dax.h b/include/linux/dax.h > > index 4b63923e1f8d..fd28d824254b 100644 > > --- a/include/linux/dax.h > > +++ b/include/linux/dax.h > > @@ -29,7 +29,7 @@ static inline struct page *read_dax_sector(struct block_device *bdev, > > } > > #endif > > > > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE > > +#if 0 > > int dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *, > > unsigned int flags, get_block_t); > > int __dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *, > > -- > > 2.6.2 > > -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
