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    SubjectRe: [PATCH 13/31] mm/hmm: retry if pte_offset_map() fails
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    On 2023/5/23 10:39, Alistair Popple wrote:
    >
    > Qi Zheng <qi.zheng@linux.dev> 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@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:
    >
    > 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.

    Yeah. There is no problem with this and the modification to
    migration_entry_wait() can be simplified. My previous thought was that
    we can finish the retry logic in hmm_vma_walk_pmd() without handing it
    over to the caller. :)

    >
    >> 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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