On 7/20/21 2:51 PM, JeffleXu wrote:
>
>
> On 7/20/21 3:44 AM, Vivek Goyal wrote:
>> On Fri, Jul 16, 2021 at 06:47:52PM +0800, Jeffle Xu wrote:
>>> Add one flag for fuse_attr.flags indicating if DAX shall be enabled for
>>> this file.
>>>
>>> When the per-file DAX flag changes for an *opened* file, the state of
>>> the file won't be updated until this file is closed and reopened later.
>>>
>>> Signed-off-by: Jeffle Xu <jefflexu@xxxxxxxxxxxxxxxxx>
>>> ---
>>> fs/fuse/dax.c | 21 +++++++++++++++++----
>>> fs/fuse/file.c | 4 ++--
>>> fs/fuse/fuse_i.h | 5 +++--
>>> fs/fuse/inode.c | 5 ++++-
>>> include/uapi/linux/fuse.h | 5 +++++
>>> 5 files changed, 31 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
>>> index a478e824c2d0..0e862119757a 100644
>>> --- a/fs/fuse/dax.c
>>> +++ b/fs/fuse/dax.c
>>> @@ -1341,7 +1341,7 @@ static const struct address_space_operations fuse_dax_file_aops = {
>>> .invalidatepage = noop_invalidatepage,
>>> };
>>>
>>> -static bool fuse_should_enable_dax(struct inode *inode)
>>> +static bool fuse_should_enable_dax(struct inode *inode, unsigned int flags)
>>> {
>>> struct fuse_conn *fc = get_fuse_conn(inode);
>>> unsigned int mode;
>>> @@ -1354,18 +1354,31 @@ static bool fuse_should_enable_dax(struct inode *inode)
>>> if (mode == FUSE_DAX_MOUNT_NEVER)
>>> return false;
>>>
>>> - return true;
>>> + if (mode == FUSE_DAX_MOUNT_ALWAYS)
>>> + return true;
>>> +
>>> + WARN_ON(mode != FUSE_DAX_MOUNT_INODE);
>>> + return flags & FUSE_ATTR_DAX;
>>> }
>>>
>>> -void fuse_dax_inode_init(struct inode *inode)
>>> +void fuse_dax_inode_init(struct inode *inode, unsigned int flags)
>>> {
>>> - if (!fuse_should_enable_dax(inode))
>>> + if (!fuse_should_enable_dax(inode, flags))
>>> return;
>>>
>>> inode->i_flags |= S_DAX;
>>> inode->i_data.a_ops = &fuse_dax_file_aops;
>>> }
>>>
>>> +void fuse_dax_dontcache(struct inode *inode, bool newdax)
>>> +{
>>> + struct fuse_conn *fc = get_fuse_conn(inode);
>>> +
>>> + if (fc->dax && fc->dax->mode == FUSE_DAX_MOUNT_INODE &&
>>> + IS_DAX(inode) != newdax)
>>> + d_mark_dontcache(inode);
>>> +}
>>> +
>>
>> This capability to mark an inode dontcache should probably be in a
>> separate patch. These seem to logically two functionalities. One is
>> enabling DAX on an inode. And second is making sure how soon you
>> see the effect of that change and hence marking inode dontcache.
>
> OK, sounds reasonable.
>
>>
>> Not sure how useful this is. In cache=none mode we should get rid of
>> inode ASAP. In cache=auto mode we will get rid of after 1 second (or
>> after a user specified timeout). So only place this seems to be
>> useful is cache=always.
>
> Actually dontcache here is used to avoid dynamic switching between DAX
> and non-DAX state while file is opened. The complexity of dynamic
> switching is that, you have to clear the address_space, since page cache
> and DAX entry can not coexist in the address space. Besides,
> inode->a_ops also needs to be changed dynamically.
FYI some context on the complication of switching DAX state dynamically,
when Ira Weiny was working on per-file DAX of ext4/xfs.
> At LSF/MM we discussed the difficulties of switching the DAX state of
a file with active mappings / page cache. It was thought the races
could be avoided by limiting DAX state flips to 0-length files.
>
> However, this turns out to not be true.[3][5] This is because address
space operations (a_ops) may be in use at any time the inode is referenced.
> from Ira Weiny
[1]
https://patchwork.kernel.org/project/xfs/cover/20200407182958.568475-1-ira.weiny@xxxxxxxxx/
[2] https://lore.kernel.org/lkml/20200305155144.GA5598@xxxxxx/
[3] https://lore.kernel.org/lkml/20200401040021.GC56958@magnolia/
[4] https://lore.kernel.org/lkml/20200403182904.GP80283@magnolia/
>
> With dontcache, dynamic switching is no longer needed and the DAX state
> will be decided only when inode (in memory) is initialized. The downside
> is that the new DAX state won't be updated until the file is closed and
> reopened later.
>
> 'cache=none' only invalidates dentry, while the inode (in memory) is
> still there (with address_space uncleared and a_ops unchanged).
>
> The dynamic switching may be done, though it's not such straightforward.
> Currently, ext4/xfs are all implemented in this dontcache way, i.e., the
> new DAX state won't be seen until the file is closed and reopened later.
>
--
Thanks,
Jeffle
