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SubjectRe: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails
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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@google.com>
> ---
> 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

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