Qi Zheng <qi.zheng@xxxxxxxxx> writes:
> On 2023/5/22 13:05, Hugh Dickins wrote:
>> hmm_vma_walk_pmd() is called through mm_walk, but already has a goto
>> again loop of its own, so take part in that if pte_offset_map() fails.
>> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
>> ---
>> mm/hmm.c | 2 ++
>> 1 file changed, 2 insertions(+)
>> diff --git a/mm/hmm.c b/mm/hmm.c
>> index e23043345615..b1a9159d7c92 100644
>> --- a/mm/hmm.c
>> +++ b/mm/hmm.c
>> @@ -381,6 +381,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>> }
>> ptep = pte_offset_map(pmdp, addr);
>> + if (!ptep)
>> + goto again;
>> for (; addr < end; addr += PAGE_SIZE, ptep++, hmm_pfns++) {
>> int r;
>>
>
> I haven't read the entire patch set yet, but taking a note here.
> The hmm_vma_handle_pte() will unmap pte and then call
> migration_entry_wait() to remap pte, so this may fail, we need to
> handle this case like below:
I don't see a problem here. Sure, hmm_vma_handle_pte() might return
-EBUSY but that will get returned up to hmm_range_fault() which will
retry the whole thing again and presumably fail when looking at the PMD.
> diff --git a/mm/hmm.c b/mm/hmm.c
> index 6a151c09de5e..eb726ff0981c 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -276,7 +276,8 @@ static int hmm_vma_handle_pte(struct mm_walk
> *walk, unsigned long addr,
> if (is_migration_entry(entry)) {
> pte_unmap(ptep);
> hmm_vma_walk->last = addr;
> - migration_entry_wait(walk->mm, pmdp, addr);
> + if (!migration_entry_wait(walk->mm, pmdp, addr))
> + return -EAGAIN;
> return -EBUSY;
> }
>
> @@ -386,6 +387,8 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
>
> r = hmm_vma_handle_pte(walk, addr, end, pmdp, ptep,
> hmm_pfns);
> if (r) {
> + if (r == -EAGAIN)
> + goto again;
> /* hmm_vma_handle_pte() did pte_unmap() */
> return r;
> }
>
> Of course, the migration_entry_wait() also needs to be modified.
