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userfaultfd/shmem: support UFFDIO_CONTINUE for shmem
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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/[email protected]
Signed-off-by: Axel Rasmussen <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Acked-by: Peter Xu <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: "Dr . David Alan Gilbert" <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Mina Almasry <[email protected]>
Cc: Oliver Upton <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Wang Qing <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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@CmdrMoozy @torvalds
CmdrMoozy authored and torvalds committed Jul 1, 2021
1 parent c949b09 commit 1531325
Showing 1 changed file with 127 additions and 45 deletions.
172 changes: 127 additions & 45 deletions mm/userfaultfd.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand All @@ -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;
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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
Expand All @@ -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);
Expand All @@ -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);
}

Expand All @@ -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:
Expand Down Expand Up @@ -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;

/*
Expand Down Expand Up @@ -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)) {
Expand Down

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