Hi Vivek,
On 5/6/23 04:37, Vivek Goyal wrote:
On Fri, May 05, 2023 at 04:16:52PM +0800, Hao Xu wrote:
From: Hao Xu <howeyxu@xxxxxxxxxxx>
FOPEN_DIRECT_IO is usually set by fuse daemon to indicate need of strong
coherency, e.g. network filesystems. Thus shared mmap is disabled since
it leverages page cache and may write to it, which may cause
inconsistence. But FOPEN_DIRECT_IO can be used not for coherency but to
reduce memory footprint as well, e.g. reduce guest memory usage with
virtiofs. Therefore, add a new flag FOPEN_DIRECT_IO_SHARED_MMAP to allow
shared mmap for these cases.
Signed-off-by: Hao Xu <howeyxu@xxxxxxxxxxx>
---
fs/fuse/file.c | 11 ++++++++---
include/uapi/linux/fuse.h | 2 ++
2 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/fs/fuse/file.c b/fs/fuse/file.c
index 89d97f6188e0..655896bdb0d5 100644
--- a/fs/fuse/file.c
+++ b/fs/fuse/file.c
@@ -161,7 +161,8 @@ struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
}
if (isdir)
- ff->open_flags &= ~FOPEN_DIRECT_IO;
+ ff->open_flags &=
+ ~(FOPEN_DIRECT_IO | FOPEN_DIRECT_IO_SHARED_MMAP);
ff->nodeid = nodeid;
@@ -2509,8 +2510,12 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
return fuse_dax_mmap(file, vma);
if (ff->open_flags & FOPEN_DIRECT_IO) {
- /* Can't provide the coherency needed for MAP_SHARED */
- if (vma->vm_flags & VM_MAYSHARE)
+ /* Can't provide the coherency needed for MAP_SHARED.
+ * So disable it if FOPEN_DIRECT_IO_SHARED_MMAP is not
+ * set, which means we do need strong coherency.
+ */
+ if (!(ff->open_flags & FOPEN_DIRECT_IO_SHARED_MMAP) &&
+ vma->vm_flags & VM_MAYSHARE)
return -ENODEV;
Can you give an example how this is useful and how do you plan to
use it?
If goal is not using guest cache (either for saving memory or for cache
coherency with other clients) and hence you used FOPEN_DIRECT_IO,
then by allowing page cache for mmap(), we are contracting that goal.
We are neither saving memory and at the same time we are not
cache coherent.
We use it to reduce guest memory "as possible as we can", which means we
first have to ensure the functionality so shared mmap should work when
users call it, then second reduce memory when users use read/write
(from/to other files).
In cases where users do read/write in most time and calls shared mmap
sometimes, disabling shared mmap makes this case out of service, but
with this flag we still reduce memory and the application works.
IIUC, for virtiofs, you want to use cache=none but at the same time
allow guest applications to do shared mmap() hence you are introducing
this change. There have been folks who have complained about it
and I think 9pfs offers a mode which does this. So solving this
problem will be nice.
BTW, if "-o dax" is used, it solves this problem. But unfortunately qemu
does not have DAX support yet and we also have issues with page truncation
on host and MCE not travelling into the guest. So DAX is not a perfect
option yet.
Yea, just like I relied in another mail, users' IO pattern may be a
bunch of small IO to a bunch of small files, dax may help but not so
much in that case.
I agree that solving this problem will be nice. Just trying to
understand the behavior better. How these cache pages will
interact with read/write?
I went through the code, it looks like there are issues when users mmap
a file and then write to it, this may cause coherency problem between
the backend file and the frontend page cache.
I think this problem exists before this patchset: when we private mmap
a file and then write to it in FOPEN_DIRECT_IO mode, the change doesn't
update to the page cache because we falsely assume there is no page
cache under FOPEN_DIRECT_IO mode. I need to go over the code and do some
test to see if it is really there and to solve it.
Thanks,
Hao
Thanks
Vivek
