From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga04-in.huawei.com (szxga04-in.huawei.com [45.249.212.190]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 05604129 for ; Thu, 14 Dec 2023 05:26:27 -0800 (PST) Received: from mail.maildlp.com (unknown [172.19.88.214]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4SrXdv3BRwz29fpL; Thu, 14 Dec 2023 21:05:39 +0800 (CST) Received: from canpemm500010.china.huawei.com (unknown [7.192.105.118]) by mail.maildlp.com (Postfix) with ESMTPS id E3A3A1A0190; Thu, 14 Dec 2023 21:06:47 +0800 (CST) Received: from [10.174.178.185] (10.174.178.185) by canpemm500010.china.huawei.com (7.192.105.118) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Thu, 14 Dec 2023 21:06:47 +0800 Subject: Re: [PATCH] ext4: fix inconsistent between segment fstrim and full fstrim To: Jan Kara References: <20231214064635.4128391-1-yebin10@huawei.com> <20231214085834.svce3mvfnctikwyq@quack3> CC: , , , From: "yebin (H)" Message-ID: <657AFDE6.1090606@huawei.com> Date: Thu, 14 Dec 2023 21:06:46 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20231214085834.svce3mvfnctikwyq@quack3> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To canpemm500010.china.huawei.com (7.192.105.118) On 2023/12/14 16:58, Jan Kara wrote: > On Thu 14-12-23 14:46:35, Ye Bin wrote: >> There will not issue discard cmd when do segment fstrim for ext4 fs, however, >> if full fstrim for the same fs will issue discard cmd. >> Above issue may happens as follows: >> Precondition: >> 1. Fstrim range [0, 15] and [16, 31]; >> 2. Discard granularity is 16; >> Range1 Range2 >> 1111000000000000 0000111010101011 >> There's no free space length large or equal than 16 in 'Range1' or 'Range2'. >> As ext4_try_to_trim_range() only search free space among range which user >> specified. However, there's maximum free space length 16 in 'Range1'+ 'Range2'. >> To solve above issue, we need to find the longest free space to discard. >> >> Signed-off-by: Ye Bin > OK, I agree that there is this behavioral difference. However is that a > practical problem? I mean I would not expect the range to be particularly > small, rather something like 1GB and then these boundary conditions don't > really matter. This is also sensible so that we can properly track whether > the whole block group was trimmed or not. Finally I'd also argue that > trimming outside of specified range might be unexpected for the user. So a > *fix* for this in my opinion lays in userspace which needs to select > sensible ranges to use for trimming. > > Honza Thanks for your reply. Our product fstrim entire file system, found to take a long time, thus affecting other processes. So they want to segment the file system fstrim based on the IO of the system. But they found that fragmented fstrims didn't work the same as fstrim for the entire file system. Users do not know the distribution of free blocks in the file system, and they do not know the reasonable range. The user's simple perception is that the effect of segmented fstrim and full fstrim should be consistent. I researched the implementation of fstrim on the XFS file system, and for the scenario described in my patch, the results of both operations are consistent. >> --- >> fs/ext4/mballoc.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c >> index d72b5e3c92ec..d195461123d8 100644 >> --- a/fs/ext4/mballoc.c >> +++ b/fs/ext4/mballoc.c >> @@ -6753,13 +6753,15 @@ static int ext4_try_to_trim_range(struct super_block *sb, >> __acquires(ext4_group_lock_ptr(sb, e4b->bd_group)) >> __releases(ext4_group_lock_ptr(sb, e4b->bd_group)) >> { >> - ext4_grpblk_t next, count, free_count; >> + ext4_grpblk_t next, count, free_count, last, origin_start; >> bool set_trimmed = false; >> void *bitmap; >> >> + last = ext4_last_grp_cluster(sb, e4b->bd_group); >> bitmap = e4b->bd_bitmap; >> - if (start == 0 && max >= ext4_last_grp_cluster(sb, e4b->bd_group)) >> + if (start == 0 && max >= last) >> set_trimmed = true; >> + origin_start = start; >> start = max(e4b->bd_info->bb_first_free, start); >> count = 0; >> free_count = 0; >> @@ -6768,7 +6770,10 @@ __releases(ext4_group_lock_ptr(sb, e4b->bd_group)) >> start = mb_find_next_zero_bit(bitmap, max + 1, start); >> if (start > max) >> break; >> - next = mb_find_next_bit(bitmap, max + 1, start); >> + >> + next = mb_find_next_bit(bitmap, last + 1, start); >> + if (origin_start == 0 && next >= last) >> + set_trimmed = true; >> >> if ((next - start) >= minblocks) { >> int ret = ext4_trim_extent(sb, start, next - start, e4b); >> -- >> 2.31.1 >>