[2/2] gfs2: Rework read and page fault locking
diff mbox series

Message ID 20200619093916.1081129-3-agruenba@redhat.com
State New
Headers show
Series
  • gfs2 readahead regression in v5.8-rc1
Related show

Commit Message

Andreas Gruenbacher June 19, 2020, 9:39 a.m. UTC
The cache consistency model of filesystems like gfs2 is such that if
data is found in the page cache, the data is up to date and can be used
without taking any filesystem locks.  If a page is not cached,
filesystem locks must be taken before populating the page cache.

Thus far,  gfs2 has taken the filesystem locks inside the ->readpage and
->readpages address space operations.  This was already causing lock
ordering problems, but commit d4388340ae0b ("fs: convert mpage_readpages
to mpage_readahead") made things worse: the ->readahead operation is
called with the pages to readahead locked, so grabbing the inode's glock
can now deadlock with processes which are holding the inode glock while
trying to lock the same pages.

Fix this by taking the inode glock in the ->read_iter file and ->fault
vm operations.  To avoid taking the inode glock when the data is already
cached, the ->read_iter file operation first tries to read the data with
the IOCB_CACHED flag set.  If that fails, the inode glock is locked and
the operation is repeated without the IOCB_CACHED flag.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
---
 fs/gfs2/aops.c | 27 ++--------------------
 fs/gfs2/file.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 61 insertions(+), 27 deletions(-)

Comments

Matthew Wilcox June 19, 2020, 11:55 a.m. UTC | #1
On Fri, Jun 19, 2020 at 11:39:16AM +0200, Andreas Gruenbacher wrote:
>  static int gfs2_readpage(struct file *file, struct page *page)
>  {
> -	struct address_space *mapping = page->mapping;
> -	struct gfs2_inode *ip = GFS2_I(mapping->host);
> -	struct gfs2_holder gh;
>  	int error;
>  
> -	unlock_page(page);
> -	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> -	error = gfs2_glock_nq(&gh);
> -	if (unlikely(error))
> -		goto out;
> -	error = AOP_TRUNCATED_PAGE;
> -	lock_page(page);
> -	if (page->mapping == mapping && !PageUptodate(page))
> -		error = __gfs2_readpage(file, page);
> -	else
> -		unlock_page(page);
> -	gfs2_glock_dq(&gh);
> -out:
> -	gfs2_holder_uninit(&gh);
> -	if (error && error != AOP_TRUNCATED_PAGE)
> +	error = __gfs2_readpage(file, page);
> +	if (error)
>  		lock_page(page);
>  	return error;

I don't think this is right.  If you return an error from ->readpage, I'm
pretty sure you're supposed to unlock that page.  Looking at
generic_file_buffered_read():

                error = mapping->a_ops->readpage(filp, page);
                if (unlikely(error)) {
                        if (error == AOP_TRUNCATED_PAGE) {
                                put_page(page);
                                error = 0;
                                goto find_page;
                        }
                        goto readpage_error;
                }
...
readpage_error:
                put_page(page);
                goto out;
...
out:
        ra->prev_pos = prev_index;
        ra->prev_pos <<= PAGE_SHIFT;
        ra->prev_pos |= prev_offset;

        *ppos = ((loff_t)index << PAGE_SHIFT) + offset;
        file_accessed(filp);
        return written ? written : error;

so we don't call unlock_page() in generic code, which means the next time
we try to get this page, we'll do ...

                page = find_get_page(mapping, index);
...
                if (!PageUptodate(page)) {
                        error = wait_on_page_locked_killable(page);
and presumably we'll wait forever because nobody is going to unlock this
page?

> @@ -598,16 +582,9 @@ static void gfs2_readahead(struct readahead_control *rac)
>  {
>  	struct inode *inode = rac->mapping->host;
>  	struct gfs2_inode *ip = GFS2_I(inode);
> -	struct gfs2_holder gh;
>  
> -	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> -	if (gfs2_glock_nq(&gh))
> -		goto out_uninit;
>  	if (!gfs2_is_stuffed(ip))
>  		mpage_readahead(rac, gfs2_block_map);
> -	gfs2_glock_dq(&gh);
> -out_uninit:
> -	gfs2_holder_uninit(&gh);
>  }

Not for this patch, obviously, but why do you go to the effort of using
iomap_readpage() to implement gfs2_readpage(), but don't use iomap for
gfs2_readahead()?  Far more pages are brought in through ->readahead
than are brought in through ->readpage.

>  static ssize_t gfs2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  {
> +	struct gfs2_inode *ip;
> +	struct gfs2_holder gh;
> +	size_t written = 0;
>  	ssize_t ret;
>  
> +	gfs2_holder_mark_uninitialized(&gh);
>  	if (iocb->ki_flags & IOCB_DIRECT) {
>  		ret = gfs2_file_direct_read(iocb, to);

Again, future work, but you probably want to pass in &gh here so you
don't have to eat up another 32 bytes or so of stack space on an unused
gfs2_holder.

>  		if (likely(ret != -ENOTBLK))
>  			return ret;
>  		iocb->ki_flags &= ~IOCB_DIRECT;
>  	}
> -	return generic_file_read_iter(iocb, to);
> +	iocb->ki_flags |= IOCB_CACHED;
> +	ret = generic_file_read_iter(iocb, to);
> +	iocb->ki_flags &= ~IOCB_CACHED;
> +	if (ret >= 0) {
> +		if (!iov_iter_count(to))
> +			return ret;
> +		written = ret;
> +	} else {
> +		switch(ret) {
> +		case -EAGAIN:
> +			if (iocb->ki_flags & IOCB_NOWAIT)
> +				return ret;
> +			break;
> +		case -ECANCELED:
> +			break;
> +		default:
> +			return ret;
> +		}
> +	}

I'm wondering if we want to do this in common code rather than making it
something special only a few filesystems do (either because they care
about workloads with many threads accessing the same file, or because
their per-file locks are very heavy-weight).

Patch
diff mbox series

diff --git a/fs/gfs2/aops.c b/fs/gfs2/aops.c
index 72c9560f4467..73c2fe768a3f 100644
--- a/fs/gfs2/aops.c
+++ b/fs/gfs2/aops.c
@@ -513,26 +513,10 @@  static int __gfs2_readpage(void *file, struct page *page)
 
 static int gfs2_readpage(struct file *file, struct page *page)
 {
-	struct address_space *mapping = page->mapping;
-	struct gfs2_inode *ip = GFS2_I(mapping->host);
-	struct gfs2_holder gh;
 	int error;
 
-	unlock_page(page);
-	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
-	error = gfs2_glock_nq(&gh);
-	if (unlikely(error))
-		goto out;
-	error = AOP_TRUNCATED_PAGE;
-	lock_page(page);
-	if (page->mapping == mapping && !PageUptodate(page))
-		error = __gfs2_readpage(file, page);
-	else
-		unlock_page(page);
-	gfs2_glock_dq(&gh);
-out:
-	gfs2_holder_uninit(&gh);
-	if (error && error != AOP_TRUNCATED_PAGE)
+	error = __gfs2_readpage(file, page);
+	if (error)
 		lock_page(page);
 	return error;
 }
@@ -598,16 +582,9 @@  static void gfs2_readahead(struct readahead_control *rac)
 {
 	struct inode *inode = rac->mapping->host;
 	struct gfs2_inode *ip = GFS2_I(inode);
-	struct gfs2_holder gh;
 
-	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
-	if (gfs2_glock_nq(&gh))
-		goto out_uninit;
 	if (!gfs2_is_stuffed(ip))
 		mpage_readahead(rac, gfs2_block_map);
-	gfs2_glock_dq(&gh);
-out_uninit:
-	gfs2_holder_uninit(&gh);
 }
 
 /**
diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index fe305e4bfd37..f729b0ff2a3c 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -558,8 +558,29 @@  static vm_fault_t gfs2_page_mkwrite(struct vm_fault *vmf)
 	return block_page_mkwrite_return(ret);
 }
 
+static vm_fault_t gfs2_fault(struct vm_fault *vmf)
+{
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	struct gfs2_inode *ip = GFS2_I(inode);
+	struct gfs2_holder gh;
+	vm_fault_t ret;
+	int err;
+
+	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
+	err = gfs2_glock_nq(&gh);
+	if (err) {
+		ret = block_page_mkwrite_return(err);
+		goto out_uninit;
+	}
+	ret = filemap_fault(vmf);
+	gfs2_glock_dq(&gh);
+out_uninit:
+	gfs2_holder_uninit(&gh);
+	return ret;
+}
+
 static const struct vm_operations_struct gfs2_vm_ops = {
-	.fault = filemap_fault,
+	.fault = gfs2_fault,
 	.map_pages = filemap_map_pages,
 	.page_mkwrite = gfs2_page_mkwrite,
 };
@@ -824,15 +845,51 @@  static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from)
 
 static ssize_t gfs2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
+	struct gfs2_inode *ip;
+	struct gfs2_holder gh;
+	size_t written = 0;
 	ssize_t ret;
 
+	gfs2_holder_mark_uninitialized(&gh);
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		ret = gfs2_file_direct_read(iocb, to);
 		if (likely(ret != -ENOTBLK))
 			return ret;
 		iocb->ki_flags &= ~IOCB_DIRECT;
 	}
-	return generic_file_read_iter(iocb, to);
+	iocb->ki_flags |= IOCB_CACHED;
+	ret = generic_file_read_iter(iocb, to);
+	iocb->ki_flags &= ~IOCB_CACHED;
+	if (ret >= 0) {
+		if (!iov_iter_count(to))
+			return ret;
+		written = ret;
+	} else {
+		switch(ret) {
+		case -EAGAIN:
+			if (iocb->ki_flags & IOCB_NOWAIT)
+				return ret;
+			break;
+		case -ECANCELED:
+			break;
+		default:
+			return ret;
+		}
+	}
+	ip = GFS2_I(iocb->ki_filp->f_mapping->host);
+	gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
+	ret = gfs2_glock_nq(&gh);
+	if (ret)
+		goto out_uninit;
+	ret = generic_file_read_iter(iocb, to);
+	if (ret > 0)
+		written += ret;
+	if (gfs2_holder_initialized(&gh))
+		gfs2_glock_dq(&gh);
+out_uninit:
+	if (gfs2_holder_initialized(&gh))
+		gfs2_holder_uninit(&gh);
+	return written ? written : ret;
 }
 
 /**