Re: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails

[Qi Zheng]

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:

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.

--
Thanks,
Qi