On Wed, Nov 18, 2015 at 4:36 PM, Ross Zwisler
<ross.zwisler@xxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 18, 2015 at 03:04:41PM -0700, Toshi Kani wrote:
>> On Wed, 2015-11-18 at 13:57 -0800, Dan Williams wrote:
>> > On Wed, Nov 18, 2015 at 1:33 PM, Toshi Kani <toshi.kani@xxxxxxx> wrote:
>> > > I am seeing a similar/same problem in my test. I think the problem is that
>> > > in
>> > > case of a WP fault, wp_huge_pmd() -> __dax_pmd_fault() ->
>> > > vmf_insert_pfn_pmd(),
>> > > which is a no-op since the PMD is mapped already. We need WP handling for
>> > > this
>> > > PMD map.
>> > >
>> > > If it helps, I have attached change for follow_trans_huge_pmd(). I have not
>> > > tested much, though.
>> >
>> > Interesting, I didn't get this far because my tests were crashing the
>> > kernel. I'll add this case the pmd fault test in ndctl.
>>
>> I hit this one with mmap(MAP_POPULATE). With this change, I then hit the WP
>> fault loop when writing to the range.
>
> Here's a fix - please let me know if this seems incomplete or incorrect for
> some reason.
>
> -- >8 --
> From 02aa9f37d7ec9c0c38413f7e304b2577eb9f974a Mon Sep 17 00:00:00 2001
> From: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Date: Wed, 18 Nov 2015 17:15:09 -0700
> Subject: [PATCH] mm: Allow DAX PMD mappings to become writeable
>
> Prior to this change DAX PMD mappings that were made read-only were never able
> to be made writable again. This is because the code in insert_pfn_pmd() that
> calls pmd_mkdirty() and pmd_mkwrite() would skip these calls if the PMD
> already existed in the page table.
>
> Instead, if we are doing a write always mark the PMD entry as dirty and
> writeable. Without this code we can get into a condition where we mark the
> PMD as read-only, and then on a subsequent write fault we get into an infinite
> loop of PMD faults where we try unsuccessfully to make the PMD writeable.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> ---
> mm/huge_memory.c | 14 ++++++--------
> 1 file changed, 6 insertions(+), 8 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index bbac913..1b3df56 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -877,15 +877,13 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
> spinlock_t *ptl;
>
> ptl = pmd_lock(mm, pmd);
> - if (pmd_none(*pmd)) {
> - entry = pmd_mkhuge(pfn_pmd(pfn, prot));
> - if (write) {
> - entry = pmd_mkyoung(pmd_mkdirty(entry));
> - entry = maybe_pmd_mkwrite(entry, vma);
> - }
> - set_pmd_at(mm, addr, pmd, entry);
> - update_mmu_cache_pmd(vma, addr, pmd);
> + entry = pmd_mkhuge(pfn_pmd(pfn, prot));
> + if (write) {
> + entry = pmd_mkyoung(pmd_mkdirty(entry));
> + entry = maybe_pmd_mkwrite(entry, vma);
> }
> + set_pmd_at(mm, addr, pmd, entry);
> + update_mmu_cache_pmd(vma, addr, pmd);
> spin_unlock(ptl);
> }
Looks good to me.
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