linux-kernel - Re: [PATCH 3/7] mm/gup: Fix the outdated code comments above get_user_pages

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Message-ID: <Z+qygOF2kueozxk0@MiWiFi-R3L-srv>
Date: Mon, 31 Mar 2025 23:19:28 +0800
From: Baoquan He <bhe@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] mm/gup: Fix the outdated code comments above
 get_user_pages_unlocked()

On 03/31/25 at 03:58pm, Oscar Salvador wrote:
> On Sun, Mar 30, 2025 at 08:17:13PM +0800, Baoquan He wrote:
> > Since commit f0818f472d8d ("mm: gup: add get_user_pages_locked
> > and get_user_pages_unlocked"), get_user_pages() doesn't need to have
> > mmap_lock held anymore. It calls __get_user_pages_locked() which
> > can acquire and drop the mmap_lock internaly.
> 
> Yes, __get_user_pages_locked() can aquire and drop the lock, but AFAICS
> get_user_pages() always calls __get_user_pages_locked() with locked=1,
> which means that is holding the lock, right?

Ah, You are right. Thanks for looking into this, Oscar. I incredibly
missed the local virable definition "int locked = 1;" line. I will drop
this patch, or wrap the code comment fix into other patch about the
obsolete reference with "get_user_pages(mm, ..., pages, NULL)".

> 
> > Hence remove the incorrect code comments now.
> > 
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > ---
> >  mm/gup.c | 16 +++-------------
> >  1 file changed, 3 insertions(+), 13 deletions(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 8788105daee8..3345a065c2cb 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2702,19 +2702,9 @@ long get_user_pages(unsigned long start, unsigned long nr_pages,
> >  EXPORT_SYMBOL(get_user_pages);
> >  
> >  /*
> > - * get_user_pages_unlocked() is suitable to replace the form:
> > - *
> > - *      mmap_read_lock(mm);
> > - *      get_user_pages(mm, ..., pages, NULL);
> > - *      mmap_read_unlock(mm);
> > - *
> > - *  with:
> > - *
> > - *      get_user_pages_unlocked(mm, ..., pages);
> > - *
> > - * It is functionally equivalent to get_user_pages_fast so
> > - * get_user_pages_fast should be used instead if specific gup_flags
> > - * (e.g. FOLL_FORCE) are not required.
> > + * get_user_pages_unlocked() is functionally equivalent to
> > + * get_user_pages_fast so get_user_pages_fast should be used instead
> > + * if specific gup_flags (e.g. FOLL_FORCE) are not required.
> >   */
> >  long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> >  			     struct page **pages, unsigned int gup_flags)
> > -- 
> > 2.41.0
> > 
> > 
> 
> -- 
> Oscar Salvador
> SUSE Labs
>