Re: [PATCH v3 7/8] ext4: Use rbtrees to manage PAs instead of inode i_prealloc_list

[Jan Kara]

Hello Ojaswin!

On Wed 08-02-23 16:55:05, Ojaswin Mujoo wrote:
> On Fri, Feb 03, 2023 at 02:06:56PM +0530, Ojaswin Mujoo wrote:
> > On Fri, Jan 27, 2023 at 03:43:12PM +0100, Jan Kara wrote:
> > > 
> > > Well, I think cond_resched() + goto retry would be OK here. We could also
> > > cycle the corresponding group lock which would wait for
> > > ext4_mb_discard_group_preallocations() to finish but that is going to burn
> > > the CPU even more than the cond_resched() + retry as we'll be just spinning
> > > on the spinlock. Sleeping is IMHO not warranted as the whole
> > > ext4_mb_discard_group_preallocations() is running under a spinlock anyway
> > > so it should better be a very short sleep.
> > > 
> > > Or actually I have one more possible solution: What the adjusting function
> > > is doing that it looks up PA before and after ac->ac_o_ex.fe_logical and
> > > trims start & end to not overlap these PAs. So we could just lookup these
> > > two PAs (ignoring the deleted state) and then just iterate from these with
> > > rb_prev() & rb_next() until we find not-deleted ones. What do you think? 
> > 
> > Hey Jan, 
> > 
> > Just thought I'd update you, I'm trying this solution out, and it looks
> > good but I'm hitting a few bugs in the implementation. Will update here
> > once I have it working correctly.
> 
> Alright, so after spending some time on these bugs I'm hitting I'm
> seeing some strange behavior. Basically, it seems like in scenarios
> where we are not able to allocate as many block as the normalized goal
> request, we can sometimes end up adding a PA that overlaps with existing
> PAs in the inode PA list/tree. This behavior exists even before this
> particular patchset. Due to presence of such overlapping PAs, the above
> logic was failing in some cases.
> 
> From my understanding of the code, this seems to be a BUG. We should not
> be adding overlapping PA ranges as that causes us to preallocate
> multiple blocks for the same logical offset in a file, however I would
> also like to know if my understanding is incorrect and if this is an
> intended behavior.
> 
> ----- Analysis of the issue ------
> 
> Here's my analysis of the behavior, which I did by adding some BUG_ONs
> and running generic/269 (4k bs). It happens pretty often, like once
> every 5-10 runs. Testing was done without applying this patch series on
> the Ted's dev branch.
> 
> 1. So taking an example of a real scenario I hit. After we find the best
> len possible, we enter the ext4_mb_new_inode_pa() function with the
> following values for start and end of the extents:
> 
> ## format: <start>/<end>(<len>)
> orig_ex:503/510(7) goal_ex:0/512(512) best_ex:0/394(394)
> 
> 2. Since (best_ex len < goal_ex len) we enter the PA window adjustment
> if condition here:
> 
> 	if (ac->ac_b_ex.fe_len < ac->ac_g_ex.fe_len)
> 		...
> 	}
> 
> 3. Here, we calc wins, winl and off and adjust logical start and end of
> the best found extent. The idea is to make sure that the best extent
> atleast covers the original request. In this example, the values are:
> 
> winl:503 wins:387 off:109
> 
> and win = min(winl, wins, off) = 109
> 
> 4. We then adjust the logical start of the best ex as:
> 
> 		ac->ac_b_ex.fe_logical = ac->ac_o_ex.fe_logical - EXT4_NUM_B2C(sbi, win);
> 
> which makes the new best extent as:
> 
> best_ex: 394/788(394)
> 
> As we can see, the best extent overflows outside the goal range, and
> hence we don't have any guarentee anymore that it will not overlap with
> another PA since we only check overlaps with the goal start and end.
> We then initialze the new PA with the logical start and end of the best
> extent and finaly add it to the inode PA list.
> 
> In my testing I was able to actually see overlapping PAs being added to
> the inode list.
> 
> ----------- END ---------------
> 
> Again, I would like to know if this is a BUG or intended. If its a BUG,
> is it okay for us to make sure the adjusted best extent length doesn't 
> extend the goal length? 

Good spotting. So I guess your understanding of mballoc is better than mine
by now :) but at least in my mental model I would also expect the resulting
preallocation to stay withing the goal extent. What is causing here the
issue is this code in ext4_mb_new_inode_pa():

                offs = ac->ac_o_ex.fe_logical %
                        EXT4_C2B(sbi, ac->ac_b_ex.fe_len);
                if (offs && offs < win)
                        win = offs;

so we apparently try to align the logical offset of the allocation to a
multiple of the allocated size but that just does not make much sense when
we found some random leftover extent with shorter-than-goal size. So what
I'd do in the shorter-than-goal preallocation case is:

1) If we can place the allocation at the end of goal window and still cover
the original allocation request, do that.

2) Otherwise if we can place the allocation at the start of the goal
window and still cover the original allocation request, do that.

3) Otherwise place the allocation at the start of the original allocation
request.

This would seem to reasonably reduce fragmentation of preallocated space
and still keep things simple.

> Also, another thing I noticed is that after ext4_mb_normalize_request(),
> sometimes the original range can also exceed the normalized goal range,
> which is again was a bit surprising to me since my understanding was
> that normalized range would always encompass the orignal range.

Well, isn't that because (part of) the requested original range is already
preallocated? Or what causes the goal range to be shortened?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR