Commit 15313257 authored by Axel Rasmussen's avatar Axel Rasmussen Committed by Linus Torvalds
Browse files

userfaultfd/shmem: support UFFDIO_CONTINUE for shmem 

With this change, userspace can resolve a minor fault within a
shmem-backed area with a UFFDIO_CONTINUE ioctl.  The semantics for this
match those for hugetlbfs - we look up the existing page in the page
cache, and install a PTE for it.

This commit introduces a new helper: mfill_atomic_install_pte.

Why handle UFFDIO_CONTINUE for shmem in mm/userfaultfd.c, instead of in
shmem.c?  The existing userfault implementation only relies on shmem.c for
VM_SHARED VMAs.  However, minor fault handling / CONTINUE work just fine
for !VM_SHARED VMAs as well.  We'd prefer to handle CONTINUE for shmem in
one place, regardless of shared/private (to reduce code duplication).

Why add a new mfill_atomic_install_pte helper?  A problem we have with
continue is that shmem_mfill_atomic_pte() and mcopy_atomic_pte() are
*close* to what we want, but not exactly.  We do want to setup the PTEs in
a CONTINUE operation, but we don't want to e.g.  allocate a new page,
charge it (e.g.  to the shmem inode), manipulate various flags, etc.  Also
we have the problem stated above: shmem_mfill_atomic_pte() and
mcopy_atomic_pte() both handle one-half of the problem (shared / private)
continue cares about.  So, introduce mcontinue_atomic_pte(), to handle all
of the shmem continue cases.  Introduce the helper so it doesn't duplicate
code with mcopy_atomic_pte().

In a future commit, shmem_mfill_atomic_pte() will also be modified to use
this new helper.  However, since this is a bigger refactor, it seems most
clear to do it as a separate change.

Link: https://lkml.kernel.org/r/20210503180737.2487560-5-axelrasmussen@google.com


Signed-off-by: default avatarAxel Rasmussen <axelrasmussen@google.com>
Acked-by: default avatarHugh Dickins <hughd@google.com>
Acked-by: default avatarPeter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Joe Perches <joe@perches.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Wang Qing <wangqing@vivo.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent c949b097

mm/userfaultfd.c

+127 −45
Original line number Diff line number Diff line
@@ -48,6 +48,83 @@ struct vm_area_struct *find_dst_vma(struct mm_struct *dst_mm, 
	return dst_vma;

}



/*

 * Install PTEs, to map dst_addr (within dst_vma) to page.

 *

 * This function handles MCOPY_ATOMIC_CONTINUE (which is always file-backed),

 * whether or not dst_vma is VM_SHARED. It also handles the more general

 * MCOPY_ATOMIC_NORMAL case, when dst_vma is *not* VM_SHARED (it may be file

 * backed, or not).

 *

 * Note that MCOPY_ATOMIC_NORMAL for a VM_SHARED dst_vma is handled by

 * shmem_mcopy_atomic_pte instead.

 */

static int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,

				    struct vm_area_struct *dst_vma,

				    unsigned long dst_addr, struct page *page,

				    bool newly_allocated, bool wp_copy)

{

	int ret;

	pte_t _dst_pte, *dst_pte;

	bool writable = dst_vma->vm_flags & VM_WRITE;

	bool vm_shared = dst_vma->vm_flags & VM_SHARED;

	bool page_in_cache = page->mapping;

	spinlock_t *ptl;

	struct inode *inode;

	pgoff_t offset, max_off;



	_dst_pte = mk_pte(page, dst_vma->vm_page_prot);

	if (page_in_cache && !vm_shared)

		writable = false;

	if (writable || !page_in_cache)

		_dst_pte = pte_mkdirty(_dst_pte);

	if (writable) {

		if (wp_copy)

			_dst_pte = pte_mkuffd_wp(_dst_pte);

		else

			_dst_pte = pte_mkwrite(_dst_pte);

	}



	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);



	if (vma_is_shmem(dst_vma)) {

		/* serialize against truncate with the page table lock */

		inode = dst_vma->vm_file->f_inode;

		offset = linear_page_index(dst_vma, dst_addr);

		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);

		ret = -EFAULT;

		if (unlikely(offset >= max_off))

			goto out_unlock;

	}



	ret = -EEXIST;

	if (!pte_none(*dst_pte))

		goto out_unlock;



	if (page_in_cache)

		page_add_file_rmap(page, false);

	else

		page_add_new_anon_rmap(page, dst_vma, dst_addr, false);



	/*

	 * Must happen after rmap, as mm_counter() checks mapping (via

	 * PageAnon()), which is set by __page_set_anon_rmap().

	 */

	inc_mm_counter(dst_mm, mm_counter(page));



	if (newly_allocated)

		lru_cache_add_inactive_or_unevictable(page, dst_vma);



	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);



	/* No need to invalidate - it was non-present before */

	update_mmu_cache(dst_vma, dst_addr, dst_pte);

	ret = 0;

out_unlock:

	pte_unmap_unlock(dst_pte, ptl);

	return ret;

}



static int mcopy_atomic_pte(struct mm_struct *dst_mm,

			    pmd_t *dst_pmd,

			    struct vm_area_struct *dst_vma,
@@ -56,13 +133,9 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm, 
			    struct page **pagep,

			    bool wp_copy)

{

	pte_t _dst_pte, *dst_pte;

	spinlock_t *ptl;

	void *page_kaddr;

	int ret;

	struct page *page;

	pgoff_t offset, max_off;

	struct inode *inode;



	if (!*pagep) {

		ret = -ENOMEM;
@@ -99,43 +172,12 @@ static int mcopy_atomic_pte(struct mm_struct *dst_mm, 
	if (mem_cgroup_charge(page, dst_mm, GFP_KERNEL))

		goto out_release;



	_dst_pte = pte_mkdirty(mk_pte(page, dst_vma->vm_page_prot));

	if (dst_vma->vm_flags & VM_WRITE) {

		if (wp_copy)

			_dst_pte = pte_mkuffd_wp(_dst_pte);

		else

			_dst_pte = pte_mkwrite(_dst_pte);

	}



	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);

	if (dst_vma->vm_file) {

		/* the shmem MAP_PRIVATE case requires checking the i_size */

		inode = dst_vma->vm_file->f_inode;

		offset = linear_page_index(dst_vma, dst_addr);

		max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);

		ret = -EFAULT;

		if (unlikely(offset >= max_off))

			goto out_release_uncharge_unlock;

	}

	ret = -EEXIST;

	if (!pte_none(*dst_pte))

		goto out_release_uncharge_unlock;



	inc_mm_counter(dst_mm, MM_ANONPAGES);

	page_add_new_anon_rmap(page, dst_vma, dst_addr, false);

	lru_cache_add_inactive_or_unevictable(page, dst_vma);



	set_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);



	/* No need to invalidate - it was non-present before */

	update_mmu_cache(dst_vma, dst_addr, dst_pte);



	pte_unmap_unlock(dst_pte, ptl);

	ret = 0;

	ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,

				       page, true, wp_copy);

	if (ret)

		goto out_release;

out:

	return ret;

out_release_uncharge_unlock:

	pte_unmap_unlock(dst_pte, ptl);

out_release:

	put_page(page);

	goto out;
@@ -176,6 +218,41 @@ static int mfill_zeropage_pte(struct mm_struct *dst_mm, 
	return ret;

}



/* Handles UFFDIO_CONTINUE for all shmem VMAs (shared or private). */

static int mcontinue_atomic_pte(struct mm_struct *dst_mm,

				pmd_t *dst_pmd,

				struct vm_area_struct *dst_vma,

				unsigned long dst_addr,

				bool wp_copy)

{

	struct inode *inode = file_inode(dst_vma->vm_file);

	pgoff_t pgoff = linear_page_index(dst_vma, dst_addr);

	struct page *page;

	int ret;



	ret = shmem_getpage(inode, pgoff, &page, SGP_READ);

	if (ret)

		goto out;

	if (!page) {

		ret = -EFAULT;

		goto out;

	}



	ret = mfill_atomic_install_pte(dst_mm, dst_pmd, dst_vma, dst_addr,

				       page, false, wp_copy);

	if (ret)

		goto out_release;



	unlock_page(page);

	ret = 0;

out:

	return ret;

out_release:

	unlock_page(page);

	put_page(page);

	goto out;

}



static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address)

{

	pgd_t *pgd;
@@ -367,11 +444,16 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm, 
						unsigned long dst_addr,

						unsigned long src_addr,

						struct page **page,

						bool zeropage,

						enum mcopy_atomic_mode mode,

						bool wp_copy)

{

	ssize_t err;



	if (mode == MCOPY_ATOMIC_CONTINUE) {

		return mcontinue_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,

					    wp_copy);

	}



	/*

	 * The normal page fault path for a shmem will invoke the

	 * fault, fill the hole in the file and COW it right away. The
@@ -383,7 +465,7 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm, 
	 * and not in the radix tree.

	 */

	if (!(dst_vma->vm_flags & VM_SHARED)) {

		if (!zeropage)

		if (mode == MCOPY_ATOMIC_NORMAL)

			err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,

					       dst_addr, src_addr, page,

					       wp_copy);
@@ -393,7 +475,8 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm, 
	} else {

		VM_WARN_ON_ONCE(wp_copy);

		err = shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,

					     dst_addr, src_addr, zeropage,

					     dst_addr, src_addr,

					     mode != MCOPY_ATOMIC_NORMAL,

					     page);

	}
@@ -415,7 +498,6 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm, 
	long copied;

	struct page *page;

	bool wp_copy;

	bool zeropage = (mcopy_mode == MCOPY_ATOMIC_ZEROPAGE);



	/*

	 * Sanitize the command parameters:
@@ -478,7 +560,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm, 


	if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma))

		goto out_unlock;

	if (mcopy_mode == MCOPY_ATOMIC_CONTINUE)

	if (!vma_is_shmem(dst_vma) && mcopy_mode == MCOPY_ATOMIC_CONTINUE)

		goto out_unlock;



	/*
@@ -526,7 +608,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm, 
		BUG_ON(pmd_trans_huge(*dst_pmd));



		err = mfill_atomic_pte(dst_mm, dst_pmd, dst_vma, dst_addr,

				       src_addr, &page, zeropage, wp_copy);

				       src_addr, &page, mcopy_mode, wp_copy);

		cond_resched();



		if (unlikely(err == -ENOENT)) {