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Message-ID: <20150916194510.GC29530@quack.suse.cz> Date: Wed, 16 Sep 2015 21:45:10 +0200 From: Jan Kara <jack@...e.cz> To: Zhang Zhen <zhenzhang.zhang@...wei.com> Cc: "tytso@....edu" <tytso@....edu>, Jan Kara <jack@...e.cz>, adilger.kernel@...ger.ca, "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org> Subject: Re: [PATCH] fs/ext4: fix potential endless loop in ext4_mb_discard_group_preallocations() On Mon 31-08-15 17:46:24, Zhang Zhen wrote: > In ext4_mb_discard_group_preallocations(), if free is always less than needed, > and some PAs are always used in every loop, it will be endless loop. > > Here we pick a random value to limit the max number of loop. Were you able to trigger this in practice or is it just a theoretical concern? My slight concern is that in theory we could prematurely declare ENOSPC with this patch since ext4_mb_discard_preallocations() doesn't reliably discard all the preallocations anymore. But probably that's acceptable. But we should add a comment before ext4_mb_discard_group_preallocations() saying that the functions needn't free all the preallocations. Honza > > Signed-off-by: Zhang Zhen <zhenzhang.zhang@...wei.com> > --- > fs/ext4/mballoc.c | 7 +++++-- > 1 file changed, 5 insertions(+), 2 deletions(-) > > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c > index 34b610e..553fbde 100644 > --- a/fs/ext4/mballoc.c > +++ b/fs/ext4/mballoc.c > @@ -3836,6 +3836,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb, > int err; > int busy = 0; > int free = 0; > + int tried = 0; > > mb_debug(1, "discard preallocation for group %u\n", group); > > @@ -3886,9 +3887,11 @@ repeat: > list_add(&pa->u.pa_tmp_list, &list); > } > > - /* if we still need more blocks and some PAs were used, try again */ > - if (free < needed && busy) { > + /* if we still need more blocks and some PAs were used, try again, > + here 20 is a ramdon value. */ > + if (free < needed && busy && tried < 20) { > busy = 0; > + tried++; > ext4_unlock_group(sb, group); > cond_resched(); > goto repeat; > -- > 1.9.1 > > > . > > > > -- Jan Kara <jack@...e.com> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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