Re: [PATCH] mm: pgtable: Ensure pml spinlock gets unlock

[I Hsin Cheng <richard120310@gmail.com>]

On Mon, Feb 10, 2025 at 12:05:05PM +0800, Qi Zheng wrote:
> 
> 
> On 2025/2/9 02:49, I Hsin Cheng wrote:
> > When !start_pte is true, the "pml" spinlock is still being holded and
> > the branch "out_pte" is taken. If "ptl" is equal to "pml", the lock
> > "pml" will still be locked when the function returns.
> 
> No. When start_pte is NULL, the ptl must also be NULL, so the ptl and
> pml will not be equal.
> 
> > 
> > It'll be better to set a new branch "out_pte" and jump to it when
> > !start_pte is true at the first place, therefore no additional check for
> > "start_pte" or "ptl != pml" is needed, simply unlock "pml" and return.
> > 
> > Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
> > ---
> >   mm/pt_reclaim.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/pt_reclaim.c b/mm/pt_reclaim.c
> > index 7e9455a18aae..163e38f1728d 100644
> > --- a/mm/pt_reclaim.c
> > +++ b/mm/pt_reclaim.c
> > @@ -43,7 +43,7 @@ void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
> >   	pml = pmd_lock(mm, pmd);
> >   	start_pte = pte_offset_map_rw_nolock(mm, pmd, addr, &pmdval, &ptl);
> >   	if (!start_pte)
> > -		goto out_ptl;
> > +		goto out_pte;
> >   	if (ptl != pml)
> >   		spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> > @@ -68,4 +68,8 @@ void try_to_free_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
> >   		pte_unmap_unlock(start_pte, ptl);
> >   	if (ptl != pml)
> >   		spin_unlock(pml);
> > +	return;
> > +
> > +out_pte:
> > +	spin_unlock(pml);
> >   }

Hi Qi,

Thanks for your kindly review!

> No. When start_pte is NULL, the ptl must also be NULL, so the ptl and
> pml will not be equal.

Since this is the case, we don't have to do any addtional check for
"start_pte" and "ptl != pml", we can be sure that only the second check
will be true so we can unlock "pml" without the redundant check, that's
my understanding, what do you think?

Best regards,
I Hsin Cheng