On Wed, Jul 31, 2024 at 1:51 PM yangerkun <yangerkun@xxxxxxxxxxxxxxx> wrote:
>
> Hi!
>
> 在 2024/7/31 19:51, Jan Kara 写道:
> > On Wed 31-07-24 12:38:35, yangerkun wrote:
> >> After we switch tmpfs dir operations from simple_dir_operations to
> >> simple_offset_dir_operations, every rename happened will fill new dentry
> >> to dest dir's maple tree(&SHMEM_I(inode)->dir_offsets->mt) with a free
> >> key starting with octx->newx_offset, and then set newx_offset equals to
> >> free key + 1. This will lead to infinite readdir combine with rename
> >> happened at the same time, which fail generic/736 in xfstests(detail show
> >> as below).
> >>
> >> 1. create 5000 files(1 2 3...) under one dir
> >> 2. call readdir(man 3 readdir) once, and get one entry
> >> 3. rename(entry, "TEMPFILE"), then rename("TEMPFILE", entry)
> >> 4. loop 2~3, until readdir return nothing or we loop too many
> >> times(tmpfs break test with the second condition)
> >>
> >> We choose the same logic what commit 9b378f6ad48cf ("btrfs: fix infinite
> >> directory reads") to fix it, record the last_index when we open dir, and
> >> do not emit the entry which index >= last_index. The file->private_data
> >> now used in offset dir can use directly to do this, and we also update
> >> the last_index when we llseek the dir file.
> >
> > The patch looks good! Just I'm not sure about the llseek part. As far as I
> > understand it was added due to this sentence in the standard:
> >
> > "If a file is removed from or added to the directory after the most recent
> > call to opendir() or rewinddir(), whether a subsequent call to readdir()
> > returns an entry for that file is unspecified."
> >
> > So if the offset used in offset_dir_llseek() is 0, then we should update
> > last_index. But otherwise I'd leave it alone because IMHO it would do more
> > harm than good.
>
> IIUC, what you means is that we should only reset the private_data to
> new last_index when we call rewinddir(which will call lseek to set
> offset of dir file to 0)?
>
> Yeah, I prefer the logic you describle! Besides, we may also change
> btrfs that do the same(e60aa5da14d0 ("btrfs: refresh dir last index
> during a rewinddir(3) call")). Filipe, how do you think?
What problem does it solve?
The standard doesn't forbid it, and I can't see anything wrong with it.
>
> Thanks,
> Erkun.
>
> > Honza
> >
> >>
> >> Fixes: a2e459555c5f ("shmem: stable directory offsets")
> >> Signed-off-by: yangerkun <yangerkun@xxxxxxxxxx>
> >> ---
> >> fs/libfs.c | 34 +++++++++++++++++++++++-----------
> >> 1 file changed, 23 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/fs/libfs.c b/fs/libfs.c
> >> index 8aa34870449f..38b306738c00 100644
> >> --- a/fs/libfs.c
> >> +++ b/fs/libfs.c
> >> @@ -450,6 +450,14 @@ void simple_offset_destroy(struct offset_ctx *octx)
> >> mtree_destroy(&octx->mt);
> >> }
> >>
> >> +static int offset_dir_open(struct inode *inode, struct file *file)
> >> +{
> >> + struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> >> +
> >> + file->private_data = (void *)ctx->next_offset;
> >> + return 0;
> >> +}
> >> +
> >> /**
> >> * offset_dir_llseek - Advance the read position of a directory descriptor
> >> * @file: an open directory whose position is to be updated
> >> @@ -463,6 +471,9 @@ void simple_offset_destroy(struct offset_ctx *octx)
> >> */
> >> static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> >> {
> >> + struct inode *inode = file->f_inode;
> >> + struct offset_ctx *ctx = inode->i_op->get_offset_ctx(inode);
> >> +
> >> switch (whence) {
> >> case SEEK_CUR:
> >> offset += file->f_pos;
> >> @@ -476,7 +487,7 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence)
> >> }
> >>
> >> /* In this case, ->private_data is protected by f_pos_lock */
> >> - file->private_data = NULL;
> >> + file->private_data = (void *)ctx->next_offset;
> >> return vfs_setpos(file, offset, LONG_MAX);
> >> }
> >>
> >> @@ -507,7 +518,7 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry)
> >> inode->i_ino, fs_umode_to_dtype(inode->i_mode));
> >> }
> >>
> >> -static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> >> +static void offset_iterate_dir(struct inode *inode, struct dir_context *ctx, long last_index)
> >> {
> >> struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode);
> >> struct dentry *dentry;
> >> @@ -515,17 +526,21 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> >> while (true) {
> >> dentry = offset_find_next(octx, ctx->pos);
> >> if (!dentry)
> >> - return ERR_PTR(-ENOENT);
> >> + return;
> >> +
> >> + if (dentry2offset(dentry) >= last_index) {
> >> + dput(dentry);
> >> + return;
> >> + }
> >>
> >> if (!offset_dir_emit(ctx, dentry)) {
> >> dput(dentry);
> >> - break;
> >> + return;
> >> }
> >>
> >> ctx->pos = dentry2offset(dentry) + 1;
> >> dput(dentry);
> >> }
> >> - return NULL;
> >> }
> >>
> >> /**
> >> @@ -552,22 +567,19 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx)
> >> static int offset_readdir(struct file *file, struct dir_context *ctx)
> >> {
> >> struct dentry *dir = file->f_path.dentry;
> >> + long last_index = (long)file->private_data;
> >>
> >> lockdep_assert_held(&d_inode(dir)->i_rwsem);
> >>
> >> if (!dir_emit_dots(file, ctx))
> >> return 0;
> >>
> >> - /* In this case, ->private_data is protected by f_pos_lock */
> >> - if (ctx->pos == DIR_OFFSET_MIN)
> >> - file->private_data = NULL;
> >> - else if (file->private_data == ERR_PTR(-ENOENT))
> >> - return 0;
> >> - file->private_data = offset_iterate_dir(d_inode(dir), ctx);
> >> + offset_iterate_dir(d_inode(dir), ctx, last_index);
> >> return 0;
> >> }
> >>
> >> const struct file_operations simple_offset_dir_operations = {
> >> + .open = offset_dir_open,
> >> .llseek = offset_dir_llseek,
> >> .iterate_shared = offset_readdir,
> >> .read = generic_read_dir,
> >> --
> >> 2.39.2
> >>
>
