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

[David Howells]

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;