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Subject[PATCH v2 6/6] gfs2: Fix mmap + page fault deadlocks for direct I/O
Date
Direct I/O differs from buffered I/O in that it uses bio_iov_iter_get_pages for
grabbing page references and for manually faulting in pages instead of
triggering actual page faults. For disabling these manual page faults, it's
not enough to call pagefault_disable(); instead, we use the new
ITER_FLAG_FAST_ONLY flag for telling iomap_dio_rw to stop faulting pages in for
us.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
fs/gfs2/file.c | 35 ++++++++++++++++++++++++++++++++++-
1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index 99df7934b4d8..6feb857a8a1c 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -763,21 +763,42 @@ static ssize_t gfs2_file_direct_read(struct kiocb *iocb, struct iov_iter *to,
struct file *file = iocb->ki_filp;
struct gfs2_inode *ip = GFS2_I(file->f_mapping->host);
size_t count = iov_iter_count(to);
+ size_t written = 0;
ssize_t ret;

+ /*
+ * In this function, we disable page faults when whe're holding the
+ * inode glock while doing I/O. If a page fault occurs, we drop the
+ * inode glock, fault in the pages manually, and then we retry. Other
+ * than in gfs2_file_read_iter, iomap_dio_rw can trigger implicit as
+ * well as manual page faults, and we need to disable both kinds
+ * separately.
+ */
+
if (!count)
return 0; /* skip atime */

gfs2_holder_init(ip->i_gl, LM_ST_DEFERRED, 0, gh);
+retry:
ret = gfs2_glock_nq(gh);
if (ret)
goto out_uninit;

+ pagefault_disable();
+ to->type |= ITER_FLAG_FAST_ONLY;
ret = iomap_dio_rw(iocb, to, &gfs2_iomap_ops, NULL, 0);
+ to->type &= ~ITER_FLAG_FAST_ONLY;
+ pagefault_enable();
+
gfs2_glock_dq(gh);
+ if (ret > 0)
+ written += ret;
+ if (unlikely(iov_iter_count(to) && (ret > 0 || ret == -EFAULT)) &&
+ fault_in_iov_iter(to))
+ goto retry;
out_uninit:
gfs2_holder_uninit(gh);
- return ret;
+ return written ? written : ret;
}

static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from,
@@ -790,6 +811,12 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from,
loff_t offset = iocb->ki_pos;
ssize_t ret;

+ /*
+ * In this function, we disable page faults when whe're holding the
+ * inode glock while doing I/O. If a page fault occurs, we drop the
+ * inode glock, fault in the pages manually, and then we retry.
+ */
+
/*
* Deferred lock, even if its a write, since we do no allocation on
* this path. All we need to change is the atime, and this lock mode
@@ -799,6 +826,7 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from,
* VFS does.
*/
gfs2_holder_init(ip->i_gl, LM_ST_DEFERRED, 0, gh);
+retry:
ret = gfs2_glock_nq(gh);
if (ret)
goto out_uninit;
@@ -807,11 +835,16 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from,
if (offset + len > i_size_read(&ip->i_inode))
goto out;

+ from->type |= ITER_FLAG_FAST_ONLY;
ret = iomap_dio_rw(iocb, from, &gfs2_iomap_ops, NULL, 0);
+ from->type &= ~ITER_FLAG_FAST_ONLY;
+
if (ret == -ENOTBLK)
ret = 0;
out:
gfs2_glock_dq(gh);
+ if (unlikely(ret == -EFAULT) && fault_in_iov_iter(from))
+ goto retry;
out_uninit:
gfs2_holder_uninit(gh);
return ret;
--
2.26.3
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