On Thu, Apr 21, 2022 at 04:04:44PM -0700, John Hubbard wrote: > On 4/21/22 09:41, Yury Norov wrote: > > Subject: [PATCH] mm/gup: fix comments to pin_user_pages_*() > > > > Hi Yuri, > > Thanks for picking this up. I have been distracted and didn't trust > myself to focus on this properly, so it's good to have help! > > IT/admin point: somehow the first line of the commit description didn't > make it into an actual email subject. The subject line was blank when it > arrived in my inbox, and the subject is in the body here instead. Not > sure how that happened. > > Maybe check your git-sendemail setup? git-sendmail is OK. I just accidentally added empty line above Subject, which broke format. My bad, sorry for this. > > pin_user_pages API forces FOLL_PIN in gup_flags, which means that the > > API requires struct page **pages to be provided (not NULL). However, > > the comment to pin_user_pages() says: > > > > * @pages: array that receives pointers to the pages pinned. > > * Should be at least nr_pages long. Or NULL, if caller > > * only intends to ensure the pages are faulted in. > > > > This patch fixes comments along the pin_user_pages code, and also adds > > WARN_ON(!pages), so that API users will have better understanding > > on how to use it. > > No need to quote the code in the commit log. Instead, just summarize. > For example: > > pin_user_pages API forces FOLL_PIN in gup_flags, which means that the > API requires struct page **pages to be provided (not NULL). However, the > comment to pin_user_pages() clearly allows for passing in a NULL @pages > argument. > > Remove the incorrect comments, and add WARN_ON_ONCE(!pages) calls to > enforce the API. > > > > > It has been independently spotted by Minchan Kim and confirmed with > > John Hubbard: > > > > https://lore.kernel.org/all/YgWA0ghrrzHONehH@xxxxxxxxxx/ > > Let's add a Cc: line for Michan as well: > > Cc: Minchan Kim <minchan@xxxxxxxxxx> He's in CC already, I think... > > Signed-off-by: Yury Norov (NVIDIA) <yury.norov@xxxxxxxxx> > > --- > > mm/gup.c | 26 ++++++++++++++++++++++---- > > 1 file changed, 22 insertions(+), 4 deletions(-) > > > > diff --git a/mm/gup.c b/mm/gup.c > > index f598a037eb04..559626457585 100644 > > --- a/mm/gup.c > > +++ b/mm/gup.c > > @@ -2871,6 +2871,10 @@ int pin_user_pages_fast(unsigned long start, int nr_pages, > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return -EINVAL; > > + /* FOLL_PIN requires pages != NULL */ > > Please delete each and every one of these one-line comments, because > they merely echo what the code says. Sure. > > + if (WARN_ON_ONCE(!pages)) > > + return -EINVAL; > > + > > gup_flags |= FOLL_PIN; > > return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages); > > } > > @@ -2893,6 +2897,10 @@ int pin_user_pages_fast_only(unsigned long start, int nr_pages, > > */ > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return 0; > > + > > + /* FOLL_PIN requires pages != NULL */ > > + if (WARN_ON_ONCE(!pages)) > > + return 0; > > /* > > * FOLL_FAST_ONLY is required in order to match the API description of > > * this routine: no fall back to regular ("slow") GUP. > > @@ -2920,8 +2928,7 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast_only); > > * @nr_pages: number of pages from start to pin > > * @gup_flags: flags modifying lookup behaviour > > * @pages: array that receives pointers to the pages pinned. > > - * Should be at least nr_pages long. Or NULL, if caller > > - * only intends to ensure the pages are faulted in. > > + * Should be at least nr_pages long. > > * @vmas: array of pointers to vmas corresponding to each page. > > * Or NULL if the caller does not require them. > > * @locked: pointer to lock flag indicating whether lock is held and > > @@ -2944,6 +2951,10 @@ long pin_user_pages_remote(struct mm_struct *mm, > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return -EINVAL; > > + /* FOLL_PIN requires pages != NULL */ > > + if (WARN_ON_ONCE(!pages)) > > + return -EINVAL; > > + > > gup_flags |= FOLL_PIN; > > return __get_user_pages_remote(mm, start, nr_pages, gup_flags, > > pages, vmas, locked); > > @@ -2957,8 +2968,7 @@ EXPORT_SYMBOL(pin_user_pages_remote); > > * @nr_pages: number of pages from start to pin > > * @gup_flags: flags modifying lookup behaviour > > * @pages: array that receives pointers to the pages pinned. > > - * Should be at least nr_pages long. Or NULL, if caller > > - * only intends to ensure the pages are faulted in. > > + * Should be at least nr_pages long. > > * @vmas: array of pointers to vmas corresponding to each page. > > * Or NULL if the caller does not require them. > > * > > @@ -2976,6 +2986,10 @@ long pin_user_pages(unsigned long start, unsigned long nr_pages, > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return -EINVAL; > > + /* FOLL_PIN requires pages != NULL */ > > + if (WARN_ON_ONCE(!pages)) > > + return -EINVAL; > > + > > gup_flags |= FOLL_PIN; > > return __gup_longterm_locked(current->mm, start, nr_pages, > > pages, vmas, gup_flags); > > @@ -2994,6 +3008,10 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages, > > if (WARN_ON_ONCE(gup_flags & FOLL_GET)) > > return -EINVAL; > > + /* FOLL_PIN requires pages != NULL */ > > + if (WARN_ON_ONCE(!pages)) > > + return -EINVAL; > > + > > gup_flags |= FOLL_PIN; > > return get_user_pages_unlocked(start, nr_pages, pages, gup_flags); > > } > > I hope we don't break any callers with the newly enforced !pages, but it's > the right thing to do, in order to avoid misunderstandings. > > thanks, > -- > John Hubbard > NVIDIA Let me test v2 and resend shortly. Thanks, Yury
