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. > 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 > -- 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
