Hello, CC to Dave, darrick.wong and xfs/nvdimm list to get more discussions. Thanks. Hao Li On 2020/8/24 14:17, Li, Hao wrote: > On 2020/8/23 14:54, Ira Weiny wrote: >> On Fri, Aug 21, 2020 at 10:40:41AM -0700, 'Ira Weiny' wrote: >>> On Fri, Aug 21, 2020 at 09:59:53AM +0800, Hao Li wrote: >>>> Currently, DCACHE_REFERENCED prevents the dentry with DCACHE_DONTCACHE >>>> set from being killed, so the corresponding inode can't be evicted. If >>>> the DAX policy of an inode is changed, we can't make policy changing >>>> take effects unless dropping caches manually. >>>> >>>> This patch fixes this problem and flushes the inode to disk to prepare >>>> for evicting it. >>> This looks intriguing and I really hope this helps but I don't think this will >>> guarantee that the state changes immediately will it? >>> >>> Do you have a test case which fails before and passes after? Perhaps one of >>> the new xfstests submitted by Xiao? >> Ok I just went back and read your comment before.[1] Sorry for being a bit >> slow on the correlation between this patch and that email. (BTW, I think it >> would have been good to put those examples in the commit message and or >> reference that example.) > Thanks for your advice. I will add those examples in v2 after further > discussion of this patch. > >> I'm assuming that with this patch example 2 from [1] >> works without a drop_cache _if_ no other task has the file open? > Yes. If no other task is opening the file, the inode and page cache of this > file will be dropped during xfs_io exiting process. There is no need to run > echo 2 > drop_caches. > >> Anyway, with that explanation I think you are correct that this improves the >> situation _if_ the only references on the file is controlled by the user and >> they have indeed closed all of them. >> >> The code for DCACHE_DONTCACHE as I attempted to write it was that it should >> have prevented further caching of the inode such that the inode would evict >> sooner. But it seems you have found a bug/optimization? > Yes. This patch is an optimization and can also be treated as a bugfix. > On the other side, even though this patch can make DCACHE_DONTCACHE more > reasonable, I am not quite sure if my approach is safe and doesn't impact > the fs performance. I hope the community can give me more advice. > >> In the end, however, if another user (such as a backup running by the admin) >> has a reference the DAX change may still be delayed. > Yes. In this situation, neither drop_caches approach nor this patch can make > the DAX change take effects soon. > Moreover, things are different if the backup task exits, this patch > will make sure the inode and page cache of the file can be dropped > _automatically_ without manual intervention. By contrast, the original > approach needs a manual cache dropping. > >> So I'm thinking the >> documentation should remain largely as is? But perhaps I am wrong. Does this >> completely remove the need for a drop_caches or only in the example you gave? > I think the contents related to drop_caches in documentation can be removed > if this patch's approach is acceptable. > >> Since I'm not a FS expert I'm still not sure. > Frankly, I'm not an expert either, so I hope this patch can be discussed > further in case it has side effects. > > Thanks, > Hao Li > >> Regardless, thanks for the fixup! :-D >> Ira >> >> [1] https://lore.kernel.org/linux-xfs/ba98b77e-a806-048a-a0dc-ca585677daf3@xxxxxxxxxxxxxx/ >> >>> Ira >>> >>>> Signed-off-by: Hao Li <lihao2018.fnst@xxxxxxxxxxxxxx> >>>> --- >>>> fs/dcache.c | 3 ++- >>>> fs/inode.c | 2 +- >>>> 2 files changed, 3 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/fs/dcache.c b/fs/dcache.c >>>> index ea0485861d93..486c7409dc82 100644 >>>> --- a/fs/dcache.c >>>> +++ b/fs/dcache.c >>>> @@ -796,7 +796,8 @@ static inline bool fast_dput(struct dentry *dentry) >>>> */ >>>> 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)) >>>> diff --git a/fs/inode.c b/fs/inode.c >>>> index 72c4c347afb7..5218a8aebd7f 100644 >>>> --- a/fs/inode.c >>>> +++ b/fs/inode.c >>>> @@ -1632,7 +1632,7 @@ static void iput_final(struct inode *inode) >>>> } >>>> >>>> state = inode->i_state; >>>> - if (!drop) { >>>> + if (!drop || (drop && (inode->i_state & I_DONTCACHE))) { >>>> WRITE_ONCE(inode->i_state, state | I_WILL_FREE); >>>> spin_unlock(&inode->i_lock); >>>> >>>> -- >>>> 2.28.0 >>>> >>>> >>>> > >
