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Subject[RFC PATCH] mm: fix device-dax pud write-faults triggered by get_user_pages()
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Currently only get_user_pages_fast() can safely handle the writable gup
case due to its use of pud_access_permitted() to check whether the pud
entry is writable. In the gup slow path pud_write() is used instead of
pud_access_permitted() and to date it has been unimplemented, just calls
BUG_ON().

kernel BUG at ./include/linux/hugetlb.h:244!
[..]
RIP: 0010:follow_devmap_pud+0x482/0x490
[..]
Call Trace:
follow_page_mask+0x28c/0x6e0
__get_user_pages+0xe4/0x6c0
get_user_pages_unlocked+0x130/0x1b0
get_user_pages_fast+0x89/0xb0
iov_iter_get_pages_alloc+0x114/0x4a0
nfs_direct_read_schedule_iovec+0xd2/0x350
? nfs_start_io_direct+0x63/0x70
nfs_file_direct_read+0x1e0/0x250
nfs_file_read+0x90/0xc0

Use pud_access_permitted() to implement pud_write(), a later cleanup can
remove {pte,pmd,pud}_write and replace them with
{pte,pmd,pud}_access_permitted() drectly so that we only have one set of
helpers these kinds of checks. For now, implementing pud_write()
simplifies -stable backports.

Cc: <stable@vger.kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Fixes: a00cc7d9dd93 ("mm, x86: add support for PUD-sized transparent hugepages")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---

Sending this as RFC for opinion on whether this should just be a
pud_flags() & _PAGE_RW check, like pmd_write, or pud_access_permitted()
that also takes protection keys into account.

include/linux/hugetlb.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index fbf5b31d47ee..6a142b240ef7 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -242,8 +242,7 @@ static inline int pgd_write(pgd_t pgd)
#ifndef pud_write
static inline int pud_write(pud_t pud)
{
- BUG();
- return 0;
+ return pud_access_permitted(pud, WRITE);
}
#endif

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