Commit a4811f53 authored by Lorenzo Stoakes's avatar Lorenzo Stoakes Committed by Andrew Morton
Browse files

mm: provide mapping_wrprotect_range() function 

In the fb_defio video driver, page dirty state is used to determine when
frame buffer pages have been changed, allowing for batched, deferred I/O
to be performed for efficiency.

This implementation had only one means of doing so effectively - the use
of the folio_mkclean() function.

However, this use of the function is inappropriate, as the fb_defio
implementation allocates kernel memory to back the framebuffer, and then
is forced to specified page->index, mapping fields in order to permit the
folio_mkclean() rmap traversal to proceed correctly.

It is not correct to specify these fields on kernel-allocated memory, and
moreover since these are not folios, page->index, mapping are deprecated
fields, soon to be removed.

We therefore need to provide a means by which we can correctly traverse
the reverse mapping and write-protect mappings for a page backing an
address_space page cache object at a given offset.

This patch provides this - mapping_wrprotect_range() - which allows for
this operation to be performed for a specified address_space, offset, PFN
and size, without requiring a folio nor, of course, an inappropriate use
of page->index, mapping.

With this provided, we can subsequently adjust the fb_defio implementation
to make use of this function and avoid incorrect invocation of
folio_mkclean() and more importantly, incorrect manipulation of
page->index and mapping fields.

Link: https://lkml.kernel.org/r/e5bf969d64e7f2f2ae944d42341fc8994b736a81.1739029358.git.lorenzo.stoakes@oracle.com


Signed-off-by: default avatarLorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Helge Deller <deller@gmx.de>
Cc: Jaya Kumar <jayakumar.lkml@gmail.com>
Cc: Kajtar Zsolt <soci@c64.rulez.org>
Cc: Maíra Canal <mcanal@igalia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Thomas Zimemrmann <tzimmermann@suse.de>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent cedae194

include/linux/rmap.h

+3 −0
Original line number Diff line number Diff line
@@ -738,6 +738,9 @@ unsigned long page_address_in_vma(const struct folio *folio, 
 */

int folio_mkclean(struct folio *);



int mapping_wrprotect_range(struct address_space *mapping, pgoff_t pgoff,

		unsigned long pfn, unsigned long nr_pages);



int pfn_mkclean_range(unsigned long pfn, unsigned long nr_pages, pgoff_t pgoff,

		      struct vm_area_struct *vma);

mm/rmap.c

+74 −0
Original line number Diff line number Diff line
@@ -1135,6 +1135,80 @@ int folio_mkclean(struct folio *folio) 
}

EXPORT_SYMBOL_GPL(folio_mkclean);



struct wrprotect_file_state {

	int cleaned;

	pgoff_t pgoff;

	unsigned long pfn;

	unsigned long nr_pages;

};



static bool mapping_wrprotect_range_one(struct folio *folio,

		struct vm_area_struct *vma, unsigned long address, void *arg)

{

	struct wrprotect_file_state *state = (struct wrprotect_file_state *)arg;

	struct page_vma_mapped_walk pvmw = {

		.pfn		= state->pfn,

		.nr_pages	= state->nr_pages,

		.pgoff		= state->pgoff,

		.vma		= vma,

		.address	= address,

		.flags		= PVMW_SYNC,

	};



	state->cleaned += page_vma_mkclean_one(&pvmw);



	return true;

}



static void __rmap_walk_file(struct folio *folio, struct address_space *mapping,

			     pgoff_t pgoff_start, unsigned long nr_pages,

			     struct rmap_walk_control *rwc, bool locked);



/**

 * mapping_wrprotect_range() - Write-protect all mappings in a specified range.

 *

 * @mapping:	The mapping whose reverse mapping should be traversed.

 * @pgoff:	The page offset at which @pfn is mapped within @mapping.

 * @pfn:	The PFN of the page mapped in @mapping at @pgoff.

 * @nr_pages:	The number of physically contiguous base pages spanned.

 *

 * Traverses the reverse mapping, finding all VMAs which contain a shared

 * mapping of the pages in the specified range in @mapping, and write-protects

 * them (that is, updates the page tables to mark the mappings read-only such

 * that a write protection fault arises when the mappings are written to).

 *

 * The @pfn value need not refer to a folio, but rather can reference a kernel

 * allocation which is mapped into userland. We therefore do not require that

 * the page maps to a folio with a valid mapping or index field, rather the

 * caller specifies these in @mapping and @pgoff.

 *

 * Return: the number of write-protected PTEs, or an error.

 */

int mapping_wrprotect_range(struct address_space *mapping, pgoff_t pgoff,

		unsigned long pfn, unsigned long nr_pages)

{

	struct wrprotect_file_state state = {

		.cleaned = 0,

		.pgoff = pgoff,

		.pfn = pfn,

		.nr_pages = nr_pages,

	};

	struct rmap_walk_control rwc = {

		.arg = (void *)&state,

		.rmap_one = mapping_wrprotect_range_one,

		.invalid_vma = invalid_mkclean_vma,

	};



	if (!mapping)

		return 0;



	__rmap_walk_file(/* folio = */NULL, mapping, pgoff, nr_pages, &rwc,

			 /* locked = */false);



	return state.cleaned;

}

EXPORT_SYMBOL_GPL(mapping_wrprotect_range);



/**

 * pfn_mkclean_range - Cleans the PTEs (including PMDs) mapped with range of

 *                     [@pfn, @pfn + @nr_pages) at the specific offset (@pgoff)