[Devel] [PATCH RHEL7 COMMIT] pfcache: do not drop pfcache checksum during mmap(SHARED) of ro-files

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-1062.12.1.vz7.131.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1062.12.1.vz7.131.5
------>
commit 8bb11ca8b200d30c5e5be10893e5af69cfde6f51
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Fri Feb 28 12:35:38 2020 +0300

    pfcache: do not drop pfcache checksum during mmap(SHARED) of ro-files
    
    Currently any mmap(SHARED) removes pfcache mark, even for
    read_only-opened files. It is required to guarantee that
    shared mappings will see changes if mapped file will be
    write-opened and modified.
    
    On the other hand we do not want to drop pfcache mark without the
    necessity. At the moment ldconfig calls mmap(SHARED) before adding
    any new files into library cache. It removes pfcache mark and
    does not allow to add libraries into pfcache.
    
    This patch helps to avoid the problem: during mmap(SHARED) for read-only
    files kernel does not drop pfcache mark from inode and still uses
    pfcache file mapping.
    
    https://jira.sw.ru/browse/PSBM-64045
    
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
    Acked-by: Konstantin Khorenko <khorenko at virtuozzo.com>
    
    v2:
    * if we do mmap(SHARED) in ro mode, we still can use pfcache file
      mapping safely because any open in rw mode will close peer (pfcache)
      file anyway, so don't close pfcache on ro mmap(SHARED) move the check
    
    * for open mode a step above, so don't need to call prep
      ext4_file_mmap(vma==NULL) if open in ro mode.
---
 mm/util.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 69dd74900ae6d..6b1cf7620e846 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -331,7 +331,8 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,
 		/* Ugly fix for PSBM-23133 vdavydov@ */
 		if (file && file->f_op && (flag & MAP_TYPE) == MAP_SHARED &&
 		    S_ISREG(file_inode(file)->i_mode) &&
-		    (file_inode(file)->i_sb->s_type->fs_flags & FS_HAS_MMAP_PREP))
+		    (file_inode(file)->i_sb->s_type->fs_flags & FS_HAS_MMAP_PREP) &&
+		    (file->f_mode & FMODE_WRITE))
 			file->f_op->mmap(file, NULL);
 		down_write(&mm->mmap_sem);
 		ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,