linux-kernel - Re: [PATCH v4] userfaultfd: opportunistic TLB-flush batching for present pages in MOVE

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Message-ID: <CAGsJ_4zK5iLtYaT2o6ctnZNUgRoxrxkDJ4gnGrTOD7CW5vuRHw@mail.gmail.com>
Date: Wed, 13 Aug 2025 17:29:28 +0800
From: Barry Song <21cnbao@...il.com>
To: Lokesh Gidra <lokeshgidra@...gle.com>
Cc: akpm@...ux-foundation.org, aarcange@...hat.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, ngeoffray@...gle.com, 
	Suren Baghdasaryan <surenb@...gle.com>, Kalesh Singh <kaleshsingh@...gle.com>, 
	Barry Song <v-songbaohua@...o.com>, David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v4] userfaultfd: opportunistic TLB-flush batching for
 present pages in MOVE

On Tue, Aug 12, 2025 at 11:50 PM Lokesh Gidra <lokeshgidra@...gle.com> wrote:

> > [...]
> >
> > >         /*
> > > @@ -1257,7 +1327,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
> > >                 if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES))
> > >                         err = -ENOENT;
> > >                 else /* nothing to do to move a hole */
> > > -                       err = 0;
> > > +                       err = PAGE_SIZE;
> >
> > To be honest, I find `err = PAGE_SIZE` quite odd :-) Could we refine the
> > code to make it more readable?
> >
> Agreed! I'll replace 'err' with 'ret' as the function no longer only
> returns error but also bytes-moved if there is no error.
>

Looks good. Should we also include the following?

--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1246,6 +1246,7 @@ static int move_zeropage_pte(struct mm_struct *mm,
 /*
  * The mmap_lock for reading is held by the caller. Just move the page(s)
  * from src_pmd to dst_pmd if possible, and return number of bytes moved.
+ * On failure, an error code is returned instead
  */
 static long move_pages_ptes(struct mm_struct *mm, pmd_t *dst_pmd,
pmd_t *src_pmd,
                            struct vm_area_struct *dst_vma,


> > [...]
> >
> > > @@ -1857,10 +1930,13 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
> > >                                 break;
> > >                         }
> > >
> > > -                       err = move_pages_pte(mm, dst_pmd, src_pmd,
> > > -                                            dst_vma, src_vma,
> > > -                                            dst_addr, src_addr, mode);
> > > -                       step_size = PAGE_SIZE;
> > > +                       ret = move_pages_ptes(mm, dst_pmd, src_pmd,
> > > +                                             dst_vma, src_vma, dst_addr,
> > > +                                             src_addr, src_end - src_addr, mode);
> > > +                       if (ret < 0)
> > > +                               err = ret;
> > > +                       else
> > > +                               step_size = ret;
> >
> > also looks a bit strange :-)
>
> Any suggestions on how to improve this? 'step_size' is expected to be
> different in each iteration of the loop even without this patch.

Usually, we have:

if (ret < 0) {
    goto or break things;
}
step_size = ret;

Given the context, it does seem quite tricky to handle. I’m not sure,
so maybe your code is fine. :-)

> >
> > >                 }
> > >
> > >                 cond_resched();
> > >
> > > base-commit: 561c80369df0733ba0574882a1635287b20f9de2
> > > --
> > > 2.50.1.703.g449372360f-goog

Thanks
Barry