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Subject[PATCH v3 1/7] iov_iter: Introduce fault_in_iov_iter helper
Date
Introduce a new fault_in_iov_iter helper for manually faulting in an iterator.
Other than fault_in_pages_writeable(), this function is non-destructive.

We'll use fault_in_iov_iter in gfs2 once we've determined that the iterator
passed to .read_iter or .write_iter isn't in memory.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
include/linux/mm.h | 3 ++
include/linux/uio.h | 1 +
lib/iov_iter.c | 42 ++++++++++++++++++++++++++++
mm/gup.c | 68 +++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 114 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7ca22e6e694a..14b1353995e2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1840,6 +1840,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
int pin_user_pages_fast(unsigned long start, int nr_pages,
unsigned int gup_flags, struct page **pages);

+unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
+ bool write);
+
int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc);
int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc,
struct task_struct *task, bool bypass_rlim);
diff --git a/include/linux/uio.h b/include/linux/uio.h
index 82c3c3e819e0..152b3605e86c 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -120,6 +120,7 @@ size_t copy_page_from_iter_atomic(struct page *page, unsigned offset,
void iov_iter_advance(struct iov_iter *i, size_t bytes);
void iov_iter_revert(struct iov_iter *i, size_t bytes);
int iov_iter_fault_in_readable(const struct iov_iter *i, size_t bytes);
+size_t fault_in_iov_iter(const struct iov_iter *i);
size_t iov_iter_single_seg_count(const struct iov_iter *i);
size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
struct iov_iter *i);
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 20dc3d800573..7221665f7ac4 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -460,6 +460,48 @@ int iov_iter_fault_in_readable(const struct iov_iter *i, size_t bytes)
}
EXPORT_SYMBOL(iov_iter_fault_in_readable);

+/**
+ * fault_in_iov_iter - fault in iov iterator for reading / writing
+ * @i: iterator
+ *
+ * Faults in the iterator using get_user_pages, i.e., without triggering
+ * hardware page faults.
+ *
+ * This is primarily useful when we know that some or all of the pages in @i
+ * aren't in memory. For iterators that are likely to be in memory,
+ * fault_in_pages_readable() may be more appropriate.
+ *
+ * Other than fault_in_pages_writeable(), this function is non-destructive even
+ * when faulting in pages for writing.
+ *
+ * Returns the number of bytes faulted in, or the size of @i if @i doesn't need
+ * faulting in.
+ */
+size_t fault_in_iov_iter(const struct iov_iter *i)
+{
+ size_t count = i->count;
+ const struct iovec *p;
+ size_t ret = 0, skip;
+
+ if (iter_is_iovec(i)) {
+ for (p = i->iov, skip = i->iov_offset; count; p++, skip = 0) {
+ unsigned long len = min(count, p->iov_len - skip);
+ unsigned long start, l;
+
+ if (unlikely(!len))
+ continue;
+ start = (unsigned long)p->iov_base + skip;
+ l = fault_in_user_pages(start, len, iov_iter_rw(i) != WRITE);
+ ret += l;
+ if (unlikely(l != len))
+ break;
+ count -= l;
+ }
+ }
+ return ret;
+}
+EXPORT_SYMBOL(fault_in_iov_iter);
+
void iov_iter_init(struct iov_iter *i, unsigned int direction,
const struct iovec *iov, unsigned long nr_segs,
size_t count)
diff --git a/mm/gup.c b/mm/gup.c
index 42b8b1fa6521..033d66586c62 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1669,6 +1669,74 @@ static long __get_user_pages_locked(struct mm_struct *mm, unsigned long start,
}
#endif /* !CONFIG_MMU */

+/**
+ * fault_in_user_pages - fault in an address range for reading / writing
+ * @start: start of address range
+ * @len: length of address range
+ * @write: fault in for writing
+ *
+ * Note that we don't pin or otherwise hold the pages referenced that we fault
+ * in. There's no guarantee that they'll stay in memory for any duration of
+ * time.
+ *
+ * Returns the number of bytes faulted in from @start.
+ */
+unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
+ bool write)
+{
+ struct mm_struct *mm = current->mm;
+ struct vm_area_struct *vma = NULL;
+ unsigned long end, nstart, nend;
+ int locked = 0;
+ int gup_flags;
+
+ /*
+ * FIXME: Make sure this function doesn't succeed for pages that cannot
+ * be accessed; otherwise we could end up in a loop trying to fault in
+ * and then access the pages. (It's okay if a page gets evicted and we
+ * need more than one retry.)
+ */
+
+ /*
+ * FIXME: Are these the right FOLL_* flags?
+ */
+
+ gup_flags = FOLL_TOUCH | FOLL_POPULATE;
+ if (write)
+ gup_flags |= FOLL_WRITE;
+
+ end = PAGE_ALIGN(start + len);
+ for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
+ unsigned long nr_pages;
+ long ret;
+
+ if (!locked) {
+ locked = 1;
+ mmap_read_lock(mm);
+ vma = find_vma(mm, nstart);
+ } else if (nstart >= vma->vm_end)
+ vma = vma->vm_next;
+ if (!vma || vma->vm_start >= end)
+ break;
+ nend = min(end, vma->vm_end);
+ if (vma->vm_flags & (VM_IO | VM_PFNMAP))
+ continue;
+ if (nstart < vma->vm_start)
+ nstart = vma->vm_start;
+ nr_pages = (nend - nstart) / PAGE_SIZE;
+ ret = __get_user_pages_locked(mm, nstart, nr_pages,
+ NULL, NULL, &locked, gup_flags);
+ if (ret <= 0)
+ break;
+ nend = nstart + ret * PAGE_SIZE;
+ }
+ if (locked)
+ mmap_read_unlock(mm);
+ if (nstart > start)
+ return min(nstart - start, len);
+ return 0;
+}
+
/**
* get_dump_page() - pin user page in memory while writing it to core dump
* @addr: user address
--
2.26.3
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