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

[Qi Zheng]

On 2023/5/23 10:39, Alistair Popple wrote:
> 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.

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