netdev - Re: [PATCH v2 14/22] netfs: New writeback implementation

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Message-ID: <458060.1714582859@warthog.procyon.org.uk>
Date: Wed, 01 May 2024 18:00:59 +0100
From: David Howells <dhowells@...hat.com>
To: Christian Brauner <christian@...uner.io>
Cc: dhowells@...hat.com, Jeff Layton <jlayton@...nel.org>,
    Gao Xiang <hsiangkao@...ux.alibaba.com>,
    Dominique Martinet <asmadeus@...ewreck.org>,
    Matthew Wilcox <willy@...radead.org>,
    Steve French <smfrench@...il.com>,
    Marc Dionne <marc.dionne@...istor.com>,
    Paulo Alcantara <pc@...guebit.com>,
    Shyam Prasad N <sprasad@...rosoft.com>, Tom Talpey <tom@...pey.com>,
    Eric Van Hensbergen <ericvh@...nel.org>,
    Ilya Dryomov <idryomov@...il.com>, netfs@...ts.linux.dev,
    linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
    linux-cifs@...r.kernel.org, linux-nfs@...r.kernel.org,
    ceph-devel@...r.kernel.org, v9fs@...ts.linux.dev,
    linux-erofs@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
    linux-mm@...ck.org, netdev@...r.kernel.org,
    linux-kernel@...r.kernel.org, Latchesar Ionkov <lucho@...kov.net>,
    Christian Schoenebeck <linux_oss@...debyte.com>
Subject: Re: [PATCH v2 14/22] netfs: New writeback implementation

This needs the attached change.  It needs to allow for netfs_perform_write()
changing i_size whilst we're doing writeback.  The issue is that i_size is
cached in the netfs_io_request struct (as that's what we're going to tell the
server the new i_size should be), but we're not updating this properly if
i_size moves between us creating the request and us deciding to write out the
folio in which i_size was when we created the request.

This can lead to the folio_zero_segment() that can be seen in the patch below
clearing the wrong amount of the final page - assuming it's still the final
page.

David
---
diff --git a/fs/netfs/write_issue.c b/fs/netfs/write_issue.c
index 69c50f4cbf41..e190043bc0da 100644
--- a/fs/netfs/write_issue.c
+++ b/fs/netfs/write_issue.c
@@ -315,13 +315,19 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 	struct netfs_group *fgroup; /* TODO: Use this with ceph */
 	struct netfs_folio *finfo;
 	size_t fsize = folio_size(folio), flen = fsize, foff = 0;
-	loff_t fpos = folio_pos(folio);
+	loff_t fpos = folio_pos(folio), i_size;
 	bool to_eof = false, streamw = false;
 	bool debug = false;
 
 	_enter("");
 
-	if (fpos >= wreq->i_size) {
+	/* netfs_perform_write() may shift i_size around the page or from out
+	 * of the page to beyond it, but cannot move i_size into or through the
+	 * page since we have it locked.
+	 */
+	i_size = i_size_read(wreq->inode);
+
+	if (fpos >= i_size) {
 		/* mmap beyond eof. */
 		_debug("beyond eof");
 		folio_start_writeback(folio);
@@ -332,6 +338,9 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 		return 0;
 	}
 
+	if (fpos + fsize > wreq->i_size)
+		wreq->i_size = i_size;
+
 	fgroup = netfs_folio_group(folio);
 	finfo = netfs_folio_info(folio);
 	if (finfo) {
@@ -342,14 +351,14 @@ static int netfs_write_folio(struct netfs_io_request *wreq,
 
 	if (wreq->origin == NETFS_WRITETHROUGH) {
 		to_eof = false;
-		if (flen > wreq->i_size - fpos)
-			flen = wreq->i_size - fpos;
-	} else if (flen > wreq->i_size - fpos) {
-		flen = wreq->i_size - fpos;
+		if (flen > i_size - fpos)
+			flen = i_size - fpos;
+	} else if (flen > i_size - fpos) {
+		flen = i_size - fpos;
 		if (!streamw)
 			folio_zero_segment(folio, flen, fsize);
 		to_eof = true;
-	} else if (flen == wreq->i_size - fpos) {
+	} else if (flen == i_size - fpos) {
 		to_eof = true;
 	}
 	flen -= foff;