Commit c4237f8b authored by John Hubbard's avatar John Hubbard Committed by Linus Torvalds
Browse files

mm: fix get_user_pages_remote()'s handling of FOLL_LONGTERM 

As it says in the updated comment in gup.c: current FOLL_LONGTERM
behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the FS
DAX check requirement on vmas.

However, the corresponding restriction in get_user_pages_remote() was
slightly stricter than is actually required: it forbade all
FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers
that do not set the "locked" arg.

Update the code and comments to loosen the restriction, allowing
FOLL_LONGTERM in some cases.

Also, copy the DAX check ("if a VMA is DAX, don't allow long term
pinning") from the VFIO call site, all the way into the internals of
get_user_pages_remote() and __gup_longterm_locked().  That is:
get_user_pages_remote() calls __gup_longterm_locked(), which in turn
calls check_dax_vmas().  This check will then be removed from the VFIO
call site in a subsequent patch.

Thanks to Jason Gunthorpe for pointing out a clean way to fix this, and
to Dan Williams for helping clarify the DAX refactoring.

Link: http://lkml.kernel.org/r/20200107224558.2362728-7-jhubbard@nvidia.com


Signed-off-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Tested-by: default avatarAlex Williamson <alex.williamson@redhat.com>
Acked-by: default avatarAlex Williamson <alex.williamson@redhat.com>
Reviewed-by: default avatarJason Gunthorpe <jgg@mellanox.com>
Reviewed-by: default avatarIra Weiny <ira.weiny@intel.com>
Suggested-by: default avatarJason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Björn Töpel <bjorn.topel@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Leon Romanovsky <leonro@mellanox.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1023369c

mm/gup.c

+92 −82
Original line number Diff line number Diff line
@@ -1111,88 +1111,6 @@ static __always_inline long __get_user_pages_locked(struct task_struct *tsk, 
	return pages_done;

}



/*

 * get_user_pages_remote() - pin user pages in memory

 * @tsk:	the task_struct to use for page fault accounting, or

 *		NULL if faults are not to be recorded.

 * @mm:		mm_struct of target mm

 * @start:	starting user address

 * @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.

 * @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

 *		subsequently whether VM_FAULT_RETRY functionality can be

 *		utilised. Lock must initially be held.

 *

 * Returns either number of pages pinned (which may be less than the

 * number requested), or an error. Details about the return value:

 *

 * -- If nr_pages is 0, returns 0.

 * -- If nr_pages is >0, but no pages were pinned, returns -errno.

 * -- If nr_pages is >0, and some pages were pinned, returns the number of

 *    pages pinned. Again, this may be less than nr_pages.

 *

 * The caller is responsible for releasing returned @pages, via put_page().

 *

 * @vmas are valid only as long as mmap_sem is held.

 *

 * Must be called with mmap_sem held for read or write.

 *

 * get_user_pages walks a process's page tables and takes a reference to

 * each struct page that each user address corresponds to at a given

 * instant. That is, it takes the page that would be accessed if a user

 * thread accesses the given user virtual address at that instant.

 *

 * This does not guarantee that the page exists in the user mappings when

 * get_user_pages returns, and there may even be a completely different

 * page there in some cases (eg. if mmapped pagecache has been invalidated

 * and subsequently re faulted). However it does guarantee that the page

 * won't be freed completely. And mostly callers simply care that the page

 * contains data that was valid *at some point in time*. Typically, an IO

 * or similar operation cannot guarantee anything stronger anyway because

 * locks can't be held over the syscall boundary.

 *

 * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page

 * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must

 * be called after the page is finished with, and before put_page is called.

 *

 * get_user_pages is typically used for fewer-copy IO operations, to get a

 * handle on the memory by some means other than accesses via the user virtual

 * addresses. The pages may be submitted for DMA to devices or accessed via

 * their kernel linear mapping (via the kmap APIs). Care should be taken to

 * use the correct cache flushing APIs.

 *

 * See also get_user_pages_fast, for performance critical applications.

 *

 * get_user_pages should be phased out in favor of

 * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing

 * should use get_user_pages because it cannot pass

 * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.

 */

long get_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)

{

	/*

	 * FIXME: Current FOLL_LONGTERM behavior is incompatible with

	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on

	 * vmas.  As there are no users of this flag in this call we simply

	 * disallow this option for now.

	 */

	if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))

		return -EINVAL;



	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,

				       locked,

				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);

}

EXPORT_SYMBOL(get_user_pages_remote);



/**

 * populate_vma_page_range() -  populate a range of pages in the vma.

 * @vma:   target vma
@@ -1626,6 +1544,98 @@ static __always_inline long __gup_longterm_locked(struct task_struct *tsk, 
}

#endif /* CONFIG_FS_DAX || CONFIG_CMA */



/*

 * get_user_pages_remote() - pin user pages in memory

 * @tsk:	the task_struct to use for page fault accounting, or

 *		NULL if faults are not to be recorded.

 * @mm:		mm_struct of target mm

 * @start:	starting user address

 * @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.

 * @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

 *		subsequently whether VM_FAULT_RETRY functionality can be

 *		utilised. Lock must initially be held.

 *

 * Returns either number of pages pinned (which may be less than the

 * number requested), or an error. Details about the return value:

 *

 * -- If nr_pages is 0, returns 0.

 * -- If nr_pages is >0, but no pages were pinned, returns -errno.

 * -- If nr_pages is >0, and some pages were pinned, returns the number of

 *    pages pinned. Again, this may be less than nr_pages.

 *

 * The caller is responsible for releasing returned @pages, via put_page().

 *

 * @vmas are valid only as long as mmap_sem is held.

 *

 * Must be called with mmap_sem held for read or write.

 *

 * get_user_pages walks a process's page tables and takes a reference to

 * each struct page that each user address corresponds to at a given

 * instant. That is, it takes the page that would be accessed if a user

 * thread accesses the given user virtual address at that instant.

 *

 * This does not guarantee that the page exists in the user mappings when

 * get_user_pages returns, and there may even be a completely different

 * page there in some cases (eg. if mmapped pagecache has been invalidated

 * and subsequently re faulted). However it does guarantee that the page

 * won't be freed completely. And mostly callers simply care that the page

 * contains data that was valid *at some point in time*. Typically, an IO

 * or similar operation cannot guarantee anything stronger anyway because

 * locks can't be held over the syscall boundary.

 *

 * If gup_flags & FOLL_WRITE == 0, the page must not be written to. If the page

 * is written to, set_page_dirty (or set_page_dirty_lock, as appropriate) must

 * be called after the page is finished with, and before put_page is called.

 *

 * get_user_pages is typically used for fewer-copy IO operations, to get a

 * handle on the memory by some means other than accesses via the user virtual

 * addresses. The pages may be submitted for DMA to devices or accessed via

 * their kernel linear mapping (via the kmap APIs). Care should be taken to

 * use the correct cache flushing APIs.

 *

 * See also get_user_pages_fast, for performance critical applications.

 *

 * get_user_pages should be phased out in favor of

 * get_user_pages_locked|unlocked or get_user_pages_fast. Nothing

 * should use get_user_pages because it cannot pass

 * FAULT_FLAG_ALLOW_RETRY to handle_mm_fault.

 */

long get_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)

{

	/*

	 * Parts of FOLL_LONGTERM behavior are incompatible with

	 * FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on

	 * vmas. However, this only comes up if locked is set, and there are

	 * callers that do request FOLL_LONGTERM, but do not set locked. So,

	 * allow what we can.

	 */

	if (gup_flags & FOLL_LONGTERM) {

		if (WARN_ON_ONCE(locked))

			return -EINVAL;

		/*

		 * This will check the vmas (even if our vmas arg is NULL)

		 * and return -ENOTSUPP if DAX isn't allowed in this case:

		 */

		return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,

					     vmas, gup_flags | FOLL_TOUCH |

					     FOLL_REMOTE);

	}



	return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas,

				       locked,

				       gup_flags | FOLL_TOUCH | FOLL_REMOTE);

}

EXPORT_SYMBOL(get_user_pages_remote);



/*

 * This is the same as get_user_pages_remote(), just with a

 * less-flexible calling convention where we assume that the task