From: Jerome Glisse <jglisse@redhat.com> To: John Hubbard <jhubbard@nvidia.com> Cc: "Andrew Morton" <akpm@linux-foundation.org>, "Al Viro" <viro@zeniv.linux.org.uk>, "Alex Williamson" <alex.williamson@redhat.com>, "Benjamin Herrenschmidt" <benh@kernel.crashing.org>, "Björn Töpel" <bjorn.topel@intel.com>, "Christoph Hellwig" <hch@infradead.org>, "Dan Williams" <dan.j.williams@intel.com>, "Daniel Vetter" <daniel@ffwll.ch>, "Dave Chinner" <david@fromorbit.com>, "David Airlie" <airlied@linux.ie>, "David S . Miller" <davem@davemloft.net>, "Ira Weiny" <ira.weiny@intel.com>, "Jan Kara" <jack@suse.cz>, "Jason Gunthorpe" <jgg@ziepe.ca>, "Jens Axboe" <axboe@kernel.dk>, "Jonathan Corbet" <corbet@lwn.net>, "Magnus Karlsson" <magnus.karlsson@intel.com>, "Mauro Carvalho Chehab" <mchehab@kernel.org>, "Michael Ellerman" <mpe@ellerman.id.au>, "Michal Hocko" <mhocko@suse.com>, "Mike Kravetz" <mike.kravetz@oracle.com>, "Paul Mackerras" <paulus@samba.org>, "Shuah Khan" <shuah@kernel.org>, "Vlastimil Babka" <vbabka@suse.cz>, bpf@vger.kernel.org, dri-devel@lists.freedesktop.org, kvm@vger.kernel.org, linux-block@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-media@vger.kernel.org, linux-rdma@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, netdev@vger.kernel.org, linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org> Subject: Re: [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN Date: Mon, 4 Nov 2019 14:18:11 -0500 Message-ID: <20191104191811.GI5134@redhat.com> (raw) In-Reply-To: <be9de35c-57e9-75c3-2e86-eae50904bbdf@nvidia.com> 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 <jglisse@redhat.com> > > [...] > >> + > >> +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
next prev parent reply index Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-03 21:17 [PATCH v2 00/18] mm/gup: track dma-pinned pages: FOLL_PIN, FOLL_LONGTERM John Hubbard 2019-11-03 21:17 ` [PATCH v2 01/18] mm/gup: pass flags arg to __gup_device_* functions John Hubbard 2019-11-04 16:39 ` Jerome Glisse 2019-11-03 21:17 ` [PATCH v2 02/18] mm/gup: factor out duplicate code from four routines John Hubbard 2019-11-04 16:51 ` Jerome Glisse 2019-11-03 21:17 ` [PATCH v2 03/18] goldish_pipe: rename local pin_user_pages() routine John Hubbard 2019-11-04 16:52 ` Jerome Glisse 2019-11-03 21:17 ` [PATCH v2 04/18] media/v4l2-core: set pages dirty upon releasing DMA buffers John Hubbard 2019-11-10 10:10 ` Hans Verkuil 2019-11-11 21:46 ` John Hubbard 2019-11-03 21:18 ` [PATCH v2 05/18] mm/gup: introduce pin_user_pages*() and FOLL_PIN John Hubbard 2019-11-04 17:33 ` Jerome Glisse 2019-11-04 19:04 ` John Hubbard 2019-11-04 19:18 ` Jerome Glisse [this message] 2019-11-04 19:30 ` John Hubbard 2019-11-04 19:52 ` Jerome Glisse 2019-11-04 20:09 ` John Hubbard 2019-11-04 20:31 ` Jason Gunthorpe 2019-11-04 20:40 ` John Hubbard 2019-11-04 20:31 ` Jerome Glisse 2019-11-04 20:37 ` Jason Gunthorpe 2019-11-04 20:57 ` John Hubbard 2019-11-04 21:15 ` Jason Gunthorpe 2019-11-04 21:34 ` John Hubbard 2019-11-04 20:33 ` David Rientjes 2019-11-04 20:48 ` Jerome Glisse 2019-11-05 13:10 ` Mike Rapoport 2019-11-05 19:00 ` John Hubbard 2019-11-07 2:25 ` Ira Weiny 2019-11-07 8:07 ` Mike Rapoport 2019-11-03 21:18 ` [PATCH v2 06/18] goldish_pipe: convert to pin_user_pages() and put_user_page() John Hubbard 2019-11-03 21:18 ` [PATCH v2 07/18] infiniband: set FOLL_PIN, FOLL_LONGTERM via pin_longterm_pages*() John Hubbard 2019-11-04 20:33 ` Jason Gunthorpe 2019-11-04 20:48 ` John Hubbard 2019-11-04 20:57 ` Jason Gunthorpe 2019-11-04 22:03 ` John Hubbard 2019-11-05 2:32 ` Jason Gunthorpe 2019-11-07 2:26 ` Ira Weiny 2019-11-03 21:18 ` [PATCH v2 08/18] mm/process_vm_access: set FOLL_PIN via pin_user_pages_remote() John Hubbard 2019-11-04 17:41 ` Jerome Glisse 2019-11-03 21:18 ` [PATCH v2 09/18] drm/via: set FOLL_PIN via pin_user_pages_fast() John Hubbard 2019-11-04 17:44 ` Jerome Glisse 2019-11-04 18:22 ` John Hubbard 2019-11-03 21:18 ` [PATCH v2 10/18] fs/io_uring: set FOLL_PIN via pin_user_pages() John Hubbard 2019-11-03 21:18 ` [PATCH v2 11/18] net/xdp: " John Hubbard 2019-11-03 21:18 ` [PATCH v2 12/18] mm/gup: track FOLL_PIN pages John Hubbard 2019-11-04 18:52 ` Jerome Glisse 2019-11-04 22:49 ` John Hubbard 2019-11-04 23:49 ` Jerome Glisse 2019-11-05 0:18 ` John Hubbard 2019-11-03 21:18 ` [PATCH v2 13/18] media/v4l2-core: pin_longterm_pages (FOLL_PIN) and put_user_page() conversion John Hubbard 2019-11-10 10:11 ` Hans Verkuil 2019-11-03 21:18 ` [PATCH v2 14/18] vfio, mm: " John Hubbard 2019-11-03 21:18 ` [PATCH v2 15/18] powerpc: book3s64: convert to pin_longterm_pages() and put_user_page() John Hubbard 2019-11-03 21:18 ` [PATCH v2 16/18] mm/gup_benchmark: support pin_user_pages() and related calls John Hubbard 2019-11-03 21:18 ` [PATCH v2 17/18] selftests/vm: run_vmtests: invoke gup_benchmark with basic FOLL_PIN coverage John Hubbard 2019-11-03 21:18 ` [PATCH v2 18/18] mm/gup: remove support for gup(FOLL_LONGTERM) John Hubbard
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