On Monday, 18 December 2023 04:34:13 CET Ira Weiny wrote:
> Fabio M. De Francesco wrote:
>
> [snip]
>
> > Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> > Signed-off-by: Fabio M. De Francesco
> > <fabio.maria.de.francesco@xxxxxxxxxxxxxxx> ---
> >
> > mm/memory.c | 5 ++---
> > 1 file changed, 2 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 7d9f6b685032..88377a107fbe 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5852,7 +5852,7 @@ static int __access_remote_vm(struct mm_struct *mm,
> > unsigned long addr,>
> > if (bytes > PAGE_SIZE-offset)
> >
> > bytes = PAGE_SIZE-offset;
> >
> > - maddr = kmap(page);
> > + maddr = kmap_local_page(page);
> >
> > if (write) {
> >
> > copy_to_user_page(vma, page, addr,
> >
> > maddr + offset, buf,
bytes);
> >
> > @@ -5861,8 +5861,7 @@ static int __access_remote_vm(struct mm_struct *mm,
> > unsigned long addr,>
> > copy_from_user_page(vma, page, addr,
> >
> > buf, maddr + offset,
bytes);
> >
> > }
> >
> > - kunmap(page);
> > - put_page(page);
> > + unmap_and_put_page(page, maddr);
>
> Does this really have the same functionality?
>
> Ira
Do you have any specific reasons to say that?
The unmap_and_put_page() helper was created by Al Viro (it initially was
put_and_unmap_page() and I sent a patch to rename it to the current name). He
noticed that we have lots of kunmap_local() followed by put_page().
The current implementation has then been changed (Matthew did it, if I
remember correctly).
My understanding of the current implementation is that unmap_and_put_page()
calls folio_release_kmap(), taking as arguments the folio which the page
belongs to and the kernel virtual address returned by kmap_local_page().
folio_release_kmap() calls kunmap_local() and then folio_put(). The last is
called on the folio obtained by the unmap_and_put_page() wrapper and, if I'm
not wrong, it releases refcounts on folios like put_page() does on pages.
Am I missing something?
For further reference, please take a look at the following path from Al Viro
that is modelled after my conversions in fs/sysv: https://lore.kernel.org/all/
20231213000849.2748576-4-viro@xxxxxxxxxxxxxxxxxx/
Thanks,
Fabio
