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Date:   Fri, 19 Feb 2021 12:34:03 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Lennert Buytenhek <buytenh@wantstofly.org>
Cc:     Jens Axboe <axboe@kernel.dk>, Al Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        io-uring@vger.kernel.org, David Laight <David.Laight@aculab.com>
Subject: Re: [PATCH v3 2/2] io_uring: add support for IORING_OP_GETDENTS
Message-ID: <20210219123403.GT2858050@casper.infradead.org>
References: <20210218122640.GA334506@wantstofly.org>
 <20210218122755.GC334506@wantstofly.org>
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On Thu, Feb 18, 2021 at 02:27:55PM +0200, Lennert Buytenhek wrote:
> IORING_OP_GETDENTS may or may not update the specified directory's
> file offset, and the file offset should not be relied upon having
> any particular value during or after an IORING_OP_GETDENTS call.

This doesn't give me the warm fuzzies.  What I might suggest
is either passing a parameter to iterate_dir() or breaking out an
iterate_dir_nofpos() to make IORING_OP_GETDENTS more of a READV operation.
ie the equivalent of this:

@@ -37,7 +37,7 @@
 } while (0)
 
 
-int iterate_dir(struct file *file, struct dir_context *ctx)
+int iterate_dir(struct file *file, struct dir_context *ctx, bool use_fpos)
 {
        struct inode *inode = file_inode(file);
        bool shared = false;
@@ -60,12 +60,14 @@ int iterate_dir(struct file *file, struct dir_context *ctx)
 
        res = -ENOENT;
        if (!IS_DEADDIR(inode)) {
-               ctx->pos = file->f_pos;
+               if (use_fpos)
+                       ctx->pos = file->f_pos;
                if (shared)
                        res = file->f_op->iterate_shared(file, ctx);
                else
                        res = file->f_op->iterate(file, ctx);
-               file->f_pos = ctx->pos;
+               if (use_fpos)
+                       file->f_pos = ctx->pos;
                fsnotify_access(file);
                file_accessed(file);
        }

That way there's no need to play with llseek or take a mutex on the
f_pos of the directory.