linux-ext4 - Re: [PATCH 2/2] ext4: skip non-loaded groups at cr=0/1

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Date:   Wed, 20 May 2020 08:40:39 +0000
From:   Alex Zhuravlev <azhuravlev@...mcloud.com>
To:     Andreas Dilger <adilger@...ger.ca>
CC:     Alex Zhuravlev <azhuravlev@...mcloud.com>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 2/2] ext4: skip non-loaded groups at cr=0/1



> On 17 May 2020, at 10:55, Andreas Dilger <adilger@...ger.ca> wrote:
> 
> The question is whether this is situation is affecting only a few inode
> allocations for a short time after mount, or does this persist for a long
> time?  I think that it _should_ be only a short time, because these other
> threads should all start prefetch on their preferred groups, so even if a
> few inodes have their blocks allocated in the "wrong" group, it shouldn't
> be a long term problem since the prefetched bitmaps will finish loading
> and allow the blocks to be allocated, or skipped if group is fragmented.

Yes, that’s the idea - there is a short window when buddy data is being
populated. And for each “cluster” (not just a single group) prefetching
will be initiated by allocation.
It’s possible that some number of inodes will get “bad” blocks right after
after mount.
If you think this is a bad scenario I can introduce couple more things:
1) few times discussed prefetching thread
2) let mballoc wait for the goal group to get ready - this essentials one
    more check in ext4_mb_good_group()
 

Thanks, Alex