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Thu, 15 Feb 2024 13:16:42 +0000 (UTC) Received: from dovecot-director2.suse.de ([10.150.64.162]) by imap2.dmz-prg2.suse.org with ESMTPSA id uHPTMLoOzmX6FwAAn2gu4w (envelope-from ); Thu, 15 Feb 2024 13:16:42 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 4B2E7A0809; Thu, 15 Feb 2024 14:16:38 +0100 (CET) Date: Thu, 15 Feb 2024 14:16:38 +0100 From: Jan Kara To: Chuck Lever Cc: viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, hughd@google.com, akpm@linux-foundation.org, Liam.Howlett@oracle.com, oliver.sang@intel.com, feng.tang@intel.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, maple-tree@lists.infradead.org, linux-mm@kvack.org, lkp@intel.com Subject: Re: [PATCH RFC 7/7] libfs: Re-arrange locking in offset_iterate_dir() Message-ID: <20240215131638.cxipaxanhidb3pev@quack3> References: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net> <170786028847.11135.14775608389430603086.stgit@91.116.238.104.host.secureserver.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <170786028847.11135.14775608389430603086.stgit@91.116.238.104.host.secureserver.net> X-Rspamd-Queue-Id: D330E8001E X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: sz1hbjahtxyb5j3zppcwytwqe8dxw7xj X-HE-Tag: 1708003004-114831 X-HE-Meta: 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 eHXwmBOF 9i65TCmQgsniewVxgMwWwN8xuzehV11m6ac59XVHErk6SwgAo/D1FGMNsxn9SY+vIOaqCME8ThJ6yZzgKzRE0DjzzyqtMZDjNip6IGeT0zqol70ayVSM2nrwPQkfc4einT2IH16dc1wHfrTQt7oW3vABEIyYXnd5AGVkHJv9rBL8VK9gfttk37WpdlFO5DrDugB+8bsT3qkzP6x1GpgRiGPkg6QNFVzVBHWUrSLQOkXxoQo5I1e9P2ayd/YV6LevvpI8r3OpKwMn5bvOXpL8L3RSxFsLElcfL4k0jSOSqcoI2KJCrzjvCNdzmSv1gZJ6XJOX8aX967fLiOFqoA1f+s+COhHJKkIpJGhBk X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue 13-02-24 16:38:08, Chuck Lever wrote: > From: Chuck Lever > > Liam says that, unlike with xarray, once the RCU read lock is > released ma_state is not safe to re-use for the next mas_find() call. > But the RCU read lock has to be released on each loop iteration so > that dput() can be called safely. > > Thus we are forced to walk the offset tree with fresh state for each > directory entry. mt_find() can do this for us, though it might be a > little less efficient than maintaining ma_state locally. > > Since offset_iterate_dir() doesn't build ma_state locally any more, > there's no longer a strong need for offset_find_next(). Clean up by > rolling these two helpers together. > > Signed-off-by: Chuck Lever Well, in general I think even xas_next_entry() is not safe to use how offset_find_next() was using it. Once you drop rcu_read_lock(), xas->xa_node could go stale. But since you're holding inode->i_rwsem when using offset_find_next() you should be protected from concurrent modifications of the mapping (whatever the underlying data structure is) - that's what makes xas_next_entry() safe AFAIU. Isn't that enough for the maple tree? Am I missing something? Honza > --- > fs/libfs.c | 39 +++++++++++++++++---------------------- > 1 file changed, 17 insertions(+), 22 deletions(-) > > diff --git a/fs/libfs.c b/fs/libfs.c > index f073e9aeb2bf..6e01fde1cf95 100644 > --- a/fs/libfs.c > +++ b/fs/libfs.c > @@ -436,23 +436,6 @@ static loff_t offset_dir_llseek(struct file *file, loff_t offset, int whence) > return vfs_setpos(file, offset, MAX_LFS_FILESIZE); > } > > -static struct dentry *offset_find_next(struct ma_state *mas) > -{ > - struct dentry *child, *found = NULL; > - > - rcu_read_lock(); > - child = mas_find(mas, ULONG_MAX); > - if (!child) > - goto out; > - spin_lock(&child->d_lock); > - if (simple_positive(child)) > - found = dget_dlock(child); > - spin_unlock(&child->d_lock); > -out: > - rcu_read_unlock(); > - return found; > -} > - > static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry) > { > unsigned long offset = dentry2offset(dentry); > @@ -465,13 +448,22 @@ static bool offset_dir_emit(struct dir_context *ctx, struct dentry *dentry) > static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx) > { > struct offset_ctx *octx = inode->i_op->get_offset_ctx(inode); > - MA_STATE(mas, &octx->mt, ctx->pos, ctx->pos); > - struct dentry *dentry; > + struct dentry *dentry, *found; > + unsigned long offset; > > + offset = ctx->pos; > while (true) { > - dentry = offset_find_next(&mas); > + found = mt_find(&octx->mt, &offset, ULONG_MAX); > + if (!found) > + goto out_noent; > + > + dentry = NULL; > + spin_lock(&found->d_lock); > + if (simple_positive(found)) > + dentry = dget_dlock(found); > + spin_unlock(&found->d_lock); > if (!dentry) > - return ERR_PTR(-ENOENT); > + goto out_noent; > > if (!offset_dir_emit(ctx, dentry)) { > dput(dentry); > @@ -479,9 +471,12 @@ static void *offset_iterate_dir(struct inode *inode, struct dir_context *ctx) > } > > dput(dentry); > - ctx->pos = mas.index + 1; > + ctx->pos = offset; > } > return NULL; > + > +out_noent: > + return ERR_PTR(-ENOENT); > } > > /** > > -- Jan Kara SUSE Labs, CR