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

[Hugh Dickins <hughd@google.com>]

On Tue, 23 May 2023, Qi Zheng wrote:
> 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. :)

Okay, Alistair has resolved this one, thanks, I agree; but what is
"the modification to migration_entry_wait()" that you refer to there?

I don't think there's any need to make it a bool, it's normal for there
to be races on entry to migration_entry_wait(), and we're used to just
returning to caller (and back up to userspace) when it does not wait.

Hugh