Re: [PATCH 03/10] fs: add namei support for doing a non-blocking path lookup

[Jens Axboe <axboe@kernel.dk>]

On 12/26/19 10:25 PM, Jens Axboe wrote:
> On 12/26/19 10:05 PM, Jens Axboe wrote:
>> On 12/26/19 5:42 PM, Al Viro wrote:
>>> On Fri, Dec 13, 2019 at 11:36:25AM -0700, Jens Axboe wrote:
>>>> If the fast lookup fails, then return -EAGAIN to have the caller retry
>>>> the path lookup. This is in preparation for supporting non-blocking
>>>> open.
>>>
>>> NAK.  We are not littering fs/namei.c with incremental broken bits
>>> and pieces with uncertain eventual use.
>>
>> To be fair, the "eventual use" is just the next patch or two...
>>
>>> And it's broken - lookup_slow() is *NOT* the only place that can and
>>> does block.  For starters, ->d_revalidate() can very well block and
>>> it is called outside of lookup_slow().  So does ->d_automount().
>>> So does ->d_manage().
>>
>> Fair enough, so it's not complete. I'd love to get it there, though!
>>
>>> I'm rather sceptical about the usefulness of non-blocking open, to be
>>> honest, but in any case, one thing that is absolutely not going to
>>> happen is piecewise introduction of such stuff without a discussion
>>> of the entire design.
>>
>> It's a necessity for io_uring, otherwise _any_ open needs to happen
>> out-of-line. But I get your objection, I'd like to get this moving in a
>> productive way though.
>>
>> What do you want it to look like? I'd be totally fine with knowing if
>> the fs has ->d_revalidate(), and always doing those out-of-line.  If I
>> know the open will be slow, that's preferable. Ditto for ->d_automount()
>> and ->d_manage(), all of that looks like cases that would be fine to
>> punt. I honestly care mostly about the cached local case _not_ needing
>> out-of-line handling, that needs to happen inline.
>>
>> Still seems to me like the LOOKUP_NONBLOCK is the way to go, and just
>> have lookup_fast() -EAGAIN if we need to call any of the potentially
>> problematic dentry ops. Yes, they _may_ not block, but they could. I
>> don't think we need to propagate this information further.
> 
> Incremental here - just check for potentially problematic dentry ops,
> and have the open redone from a path where it doesn't matter.

Here's the (updated) full patch, with the bits cleaned up a bit. Would
this be more agreeable to you?


commit ac605d1d6ca445ba7e2990e0afe0e28ad831a663
Author: Jens Axboe <axboe@kernel.dk>
Date:   Fri Dec 13 11:09:26 2019 -0700

    fs: add namei support for doing a non-blocking path lookup
    
    If the fast lookup fails, then return -EAGAIN to have the caller retry
    the path lookup. Assume that a dentry having any of:
    
    ->d_revalidate()
    ->d_automount()
    ->d_manage()
    
    could block in those callbacks. Preemptively return -EAGAIN if any of
    these are present.
    
    This is in preparation for supporting non-blocking open.
    
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/namei.c b/fs/namei.c
index d6c91d1e88cb..2bfdb932f2f2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1549,6 +1549,17 @@ static struct dentry *__lookup_hash(const struct qstr *name,
 	return dentry;
 }
 
+static inline bool lookup_could_block(struct dentry *dentry, unsigned int flags)
+{
+	const struct dentry_operations *ops = dentry->d_op;
+
+	if (!(flags & LOOKUP_NONBLOCK))
+		return 0;
+
+	/* assume these dentry ops may block */
+	return ops->d_revalidate || ops->d_automount || ops->d_manage;
+}
+
 static int lookup_fast(struct nameidata *nd,
 		       struct path *path, struct inode **inode,
 		       unsigned *seqp)
@@ -1573,6 +1584,9 @@ static int lookup_fast(struct nameidata *nd,
 			return 0;
 		}
 
+		if (unlikely(lookup_could_block(dentry, nd->flags)))
+			return -EAGAIN;
+
 		/*
 		 * This sequence count validates that the inode matches
 		 * the dentry name information from lookup.
@@ -1615,7 +1629,10 @@ static int lookup_fast(struct nameidata *nd,
 		dentry = __d_lookup(parent, &nd->last);
 		if (unlikely(!dentry))
 			return 0;
-		status = d_revalidate(dentry, nd->flags);
+		if (unlikely(lookup_could_block(dentry, nd->flags)))
+			status = -EAGAIN;
+		else
+			status = d_revalidate(dentry, nd->flags);
 	}
 	if (unlikely(status <= 0)) {
 		if (!status)
@@ -1799,6 +1816,8 @@ static int walk_component(struct nameidata *nd, int flags)
 	if (unlikely(err <= 0)) {
 		if (err < 0)
 			return err;
+		if (nd->flags & LOOKUP_NONBLOCK)
+			return -EAGAIN;
 		path.dentry = lookup_slow(&nd->last, nd->path.dentry,
 					  nd->flags);
 		if (IS_ERR(path.dentry))
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 7fe7b87a3ded..935a1bf0caca 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -38,6 +38,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_JUMPED		0x1000
 #define LOOKUP_ROOT		0x2000
 #define LOOKUP_ROOT_GRABBED	0x0008
+#define LOOKUP_NONBLOCK		0x10000	/* don't block for lookup */
 
 extern int path_pts(struct path *path);

-- 
Jens Axboe