On Thu, Sep 24, 2020 at 01:59:58PM +0800, Hao Li wrote: > If DCACHE_REFERENCED is set, fast_dput() will return true, and then > retain_dentry() have no chance to check DCACHE_DONTCACHE. As a result, > the dentry won't be killed and the corresponding inode can't be evicted. > In the following example, the DAX policy can't take effects unless we > do a drop_caches manually. > > # DCACHE_LRU_LIST will be set > echo abcdefg > test.txt > > # DCACHE_REFERENCED will be set and DCACHE_DONTCACHE can't do anything > xfs_io -c 'chattr +x' test.txt > > # Drop caches to make DAX changing take effects > echo 2 > /proc/sys/vm/drop_caches > > What this patch does is preventing fast_dput() from returning true if > DCACHE_DONTCACHE is set. Then retain_dentry() will detect the > DCACHE_DONTCACHE and will return false. As a result, the dentry will be > killed and the inode will be evicted. In this way, if we change per-file > DAX policy, it will take effects automatically after this file is closed > by all processes. > > I also add some comments to make the code more clear. > > Signed-off-by: Hao Li <lihao2018.fnst@xxxxxxxxxxxxxx> Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx> > --- > v1 is split into two standalone patch as discussed in [1], and the first > patch has been reviewed in [2]. This is the second patch. > > [1]: https://lore.kernel.org/linux-fsdevel/20200831003407.GE12096@xxxxxxxxxxxxxxxxxxx/ > [2]: https://lore.kernel.org/linux-fsdevel/20200906214002.GI12131@xxxxxxxxxxxxxxxxxxx/ > > fs/dcache.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/fs/dcache.c b/fs/dcache.c > index ea0485861d93..97e81a844a96 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -793,10 +793,17 @@ static inline bool fast_dput(struct dentry *dentry) > * a reference to the dentry and change that, but > * our work is done - we can leave the dentry > * around with a zero refcount. > + * > + * Nevertheless, there are two cases that we should kill > + * the dentry anyway. > + * 1. free disconnected dentries as soon as their refcount > + * reached zero. > + * 2. free dentries if they should not be cached. > */ > smp_rmb(); > d_flags = READ_ONCE(dentry->d_flags); > - d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED; > + d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | > + DCACHE_DISCONNECTED | DCACHE_DONTCACHE; > > /* Nothing to do? Dropping the reference was all we needed? */ > if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry)) > -- > 2.28.0 > > >
