Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN

From: Jerome Glisse <[email-masked]>
Date: Mon, 4 Nov 2019 14:18:11 -0500

On Mon, Nov 04, 2019 at 11:04:38AM -0800, John Hubbard wrote:
> On 11/4/19 9:33 AM, Jerome Glisse wrote:
> ...
> > 
> > Few nitpick belows, nonetheless:
> > 
> > Reviewed-by: Jérôme Glisse <[email-masked]>
> > [...]
> >> +
> >> +CASE 3: ODP
> >> +-----------
> >> +(Mellanox/Infiniband On Demand Paging: the hardware supports
> >> +replayable page faulting). There are GUP references to pages serving as DMA
> >> +buffers. For ODP, MMU notifiers are used to synchronize with page_mkclean()
> >> +and munmap(). Therefore, normal GUP calls are sufficient, so neither flag
> >> +needs to be set.
> > 
> > I would not include ODP or anything like it here, they do not use
> > GUP anymore and i believe it is more confusing here. I would how-
> > ever include some text in this documentation explaining that hard-
> > ware that support page fault is superior as it does not incur any
> > of the issues described here.
> 
> OK, agreed, here's a new write up that I'll put in v3:
> 
> 
> CASE 3: ODP
> -----------

ODP is RDMA, maybe Hardware with page fault support instead

> Advanced, but non-CPU (DMA) hardware that supports replayable page faults.
> Here, a well-written driver doesn't normally need to pin pages at all. However,
> if the driver does choose to do so, it can register MMU notifiers for the range,
> and will be called back upon invalidation. Either way (avoiding page pinning, or
> using MMU notifiers to unpin upon request), there is proper synchronization with 
> both filesystem and mm (page_mkclean(), munmap(), etc).
> 
> Therefore, neither flag needs to be set.

In fact GUP should never be use with those.

> 
> It's worth mentioning here that pinning pages should not be the first design
> choice. If page fault capable hardware is available, then the software should
> be written so that it does not pin pages. This allows mm and filesystems to
> operate more efficiently and reliably.
> 
> > [...]
> > 
> >> diff --git a/mm/gup.c b/mm/gup.c
> >> index 199da99e8ffc..1aea48427879 100644
> >> --- a/mm/gup.c
> >> +++ b/mm/gup.c
> > 
> > [...]
> > 
> >> @@ -1014,7 +1018,16 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
> >>  		BUG_ON(*locked != 1);
> >>  	}
> >>  
> >> -	if (pages)
> >> +	/*
> >> +	 * FOLL_PIN and FOLL_GET are mutually exclusive. Traditional behavior
> >> +	 * is to set FOLL_GET if the caller wants pages[] filled in (but has
> >> +	 * carelessly failed to specify FOLL_GET), so keep doing that, but only
> >> +	 * for FOLL_GET, not for the newer FOLL_PIN.
> >> +	 *
> >> +	 * FOLL_PIN always expects pages to be non-null, but no need to assert
> >> +	 * that here, as any failures will be obvious enough.
> >> +	 */
> >> +	if (pages && !(flags & FOLL_PIN))
> >>  		flags |= FOLL_GET;
> > 
> > Did you look at user that have pages and not FOLL_GET set ?
> > I believe it would be better to first fix them to end up
> > with FOLL_GET set and then error out if pages is != NULL but
> > nor FOLL_GET or FOLL_PIN is set.
> > 
> 
> I was perhaps overly cautious, and didn't go there. However, it's probably
> doable, given that there was already the following in __get_user_pages():
> 
>     VM_BUG_ON(!!pages != !!(gup_flags & FOLL_GET));
> 
> ...which will have conditioned people and code to set FOLL_GET together with
> pages. So I agree that the time is right.
> 
> In order to make bisecting future failures simpler, I can insert a patch right 
> before this one, that changes the FOLL_GET setting into an assert, like this:
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8f236a335ae9..be338961e80d 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1014,8 +1014,8 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk,
>                 BUG_ON(*locked != 1);
>         }
>  
> -       if (pages)
> -               flags |= FOLL_GET;
> +       if (pages && WARN_ON_ONCE(!(gup_flags & FOLL_GET)))
> +               return -EINVAL;
>  
>         pages_done = 0;
>         lock_dropped = false;
> 
> 
> ...and then add in FOLL_PIN, with this patch.

looks good but double check that it should not happens, i will try
to check on my side too.

> 
> >>  
> >>  	pages_done = 0;
> > 
> >> @@ -2373,24 +2402,9 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
> >>  	return ret;
> >>  }
> >>  
> >> -/**
> >> - * get_user_pages_fast() - pin user pages in memory
> >> - * @start:	starting user address
> >> - * @nr_pages:	number of pages from start to pin
> >> - * @gup_flags:	flags modifying pin behaviour
> >> - * @pages:	array that receives pointers to the pages pinned.
> >> - *		Should be at least nr_pages long.
> >> - *
> >> - * Attempt to pin user pages in memory without taking mm->mmap_sem.
> >> - * If not successful, it will fall back to taking the lock and
> >> - * calling get_user_pages().
> >> - *
> >> - * Returns number of pages pinned. This may be fewer than the number
> >> - * requested. If nr_pages is 0 or negative, returns 0. If no pages
> >> - * were pinned, returns -errno.
> >> - */
> >> -int get_user_pages_fast(unsigned long start, int nr_pages,
> >> -			unsigned int gup_flags, struct page **pages)
> >> +static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
> >> +					unsigned int gup_flags,
> >> +					struct page **pages)
> > 
> > Usualy function are rename to _old_func_name ie add _ in front. So
> > here it would become _get_user_pages_fast but i know some people
> > don't like that as sometimes we endup with ___function_overloaded :)
> 
> Exactly: the __get_user_pages* names were already used for *non*-internal
> routines, so I attempted to pick the next best naming prefix.

Didn't know we were that far in the ___ :)

> > 
> >>  {
> >>  	unsigned long addr, len, end;
> >>  	int nr = 0, ret = 0;
> > 
> > 
> >> @@ -2435,4 +2449,215 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> > 
> > [...]
> > 
> >> +/**
> >> + * pin_user_pages_remote() - pin pages for (typically) use by Direct IO, and
> >> + * return the pages to the user.
> > 
> > Not a fan of (typically) maybe:
> > pin_user_pages_remote() - pin pages of a remote process (task != current)
> > 
> > I think here the remote part if more important that DIO. Remote is use by
> > other thing that DIO.
> 
> Yes, good point. I'll use your wording:
> 
>  * pin_user_pages_remote() - pin pages of a remote process (task != current)
> 
> 
> 
> > 
> >> + *
> >> + * Nearly the same as get_user_pages_remote(), except that FOLL_PIN is set. See
> >> + * get_user_pages_remote() for documentation on the function arguments, because
> >> + * the arguments here are identical.
> >> + *
> >> + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> >> + * see Documentation/vm/pin_user_pages.rst for details.
> >> + *
> >> + * This is intended for Case 1 (DIO) in Documentation/vm/pin_user_pages.rst. It
> >> + * is NOT intended for Case 2 (RDMA: long-term pins).
> >> + */
> >> +long pin_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
> >> +			   unsigned long start, unsigned long nr_pages,
> >> +			   unsigned int gup_flags, struct page **pages,
> >> +			   struct vm_area_struct **vmas, int *locked)
> >> +{
> >> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> >> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> >> +		return -EINVAL;
> >> +
> >> +	gup_flags |= FOLL_TOUCH | FOLL_REMOTE | FOLL_PIN;
> >> +
> >> +	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,
> >> +				       locked, gup_flags);
> >> +}
> >> +EXPORT_SYMBOL(pin_user_pages_remote);
> >> +
> >> +/**
> >> + * pin_longterm_pages_remote() - pin pages for (typically) use by Direct IO, and
> >> + * return the pages to the user.
> > 
> > I think you copy pasted this from pin_user_pages_remote() :)
> 
> I admit to nothing, with respect to copy-paste! :)
> 
> This one can simply be:
> 
>  * pin_longterm_pages_remote() - pin pages of a remote process (task != current)
> 
> 
> > 
> >> + *
> >> + * Nearly the same as get_user_pages_remote(), but note that FOLL_TOUCH is not
> >> + * set, and FOLL_PIN and FOLL_LONGTERM are set. See get_user_pages_remote() for
> >> + * documentation on the function arguments, because the arguments here are
> >> + * identical.
> >> + *
> >> + * FOLL_PIN means that the pages must be released via put_user_page(). Please
> >> + * see Documentation/vm/pin_user_pages.rst for further details.
> >> + *
> >> + * FOLL_LONGTERM means that the pages are being pinned for "long term" use,
> >> + * typically by a non-CPU device, and we cannot be sure that waiting for a
> >> + * pinned page to become unpin will be effective.
> >> + *
> >> + * This is intended for Case 2 (RDMA: long-term pins) in
> >> + * Documentation/vm/pin_user_pages.rst.
> >> + */
> >> +long pin_longterm_pages_remote(struct task_struct *tsk, struct mm_struct *mm,
> >> +			       unsigned long start, unsigned long nr_pages,
> >> +			       unsigned int gup_flags, struct page **pages,
> >> +			       struct vm_area_struct **vmas, int *locked)
> >> +{
> >> +	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
> >> +	if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> >> +		return -EINVAL;
> >> +
> >> +	/*
> >> +	 * FIXME: as noted in the get_user_pages_remote() implementation, it
> >> +	 * is not yet possible to safely set FOLL_LONGTERM here. FOLL_LONGTERM
> >> +	 * needs to be set, but for now the best we can do is a "TODO" item.
> >> +	 */
> >> +	gup_flags |= FOLL_REMOTE | FOLL_PIN;
> > 
> > Wouldn't it be better to not add pin_longterm_pages_remote() until
> > it can be properly implemented ?
> > 
> 
> Well, the problem is that I need each call site that requires FOLL_PIN
> to use a proper wrapper. It's the FOLL_PIN that is the focus here, because
> there is a hard, bright rule, which is: if and only if a caller sets
> FOLL_PIN, then the dma-page tracking happens, and put_user_page() must
> be called.
> 
> So this leaves me with only two reasonable choices:
> 
> a) Convert the call site as above: pin_longterm_pages_remote(), which sets
> FOLL_PIN (the key point!), and leaves the FOLL_LONGTERM situation exactly
> as it has been so far. When the FOLL_LONGTERM situation is fixed, the call
> site *might* not need any changes to adopt the working gup.c code.
> 
> b) Convert the call site to pin_user_pages_remote(), which also sets
> FOLL_PIN, and also leaves the FOLL_LONGTERM situation exactly as before.
> There would also be a comment at the call site, to the effect of, "this
> is the wrong call to make: it really requires FOLL_LONGTERM behavior".
> 
> When the FOLL_LONGTERM situation is fixed, the call site will need to be
> changed to pin_longterm_pages_remote().
> 
> So you can probably see why I picked (a).

But right now nobody has FOLL_LONGTERM and FOLL_REMOTE. So you should
never have the need for pin_longterm_pages_remote(). My fear is that
longterm has implication and it would be better to not drop this implication
by adding a wrapper that does not do what the name says.

So do not introduce pin_longterm_pages_remote() until its first user
happens. This is option c)

Cheers,
Jérôme