linux-kernel - Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation mode mount option

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Date:   Fri, 10 Sep 2021 08:24:42 -0700
From:   Daeho Jeong <daeho43@...il.com>
To:     Chao Yu <chao@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@...roid.com,
        Daeho Jeong <daehojeong@...gle.com>
Subject: Re: [f2fs-dev] [PATCH v4] f2fs: introduce fragment allocation mode
 mount option

On Fri, Sep 10, 2021 at 7:34 AM Daeho Jeong <daeho43@...il.com> wrote:
>
> On Thu, Sep 9, 2021 at 4:50 PM Chao Yu <chao@...nel.org> wrote:
> >
> > On 2021/9/8 2:12, Daeho Jeong wrote:
> > > On Fri, Sep 3, 2021 at 11:45 PM Chao Yu <chao@...nel.org> wrote:
> > >>
> > >> On 2021/9/4 12:40, Daeho Jeong wrote:
> > >>>> As a per curseg field.
> > >>>>
> > >>>>> Maybe, we run into the same race condition issue you told before for
> > >>>>> fragment_remained_chunk.
> > >>>>> Could you clarify this more?
> > >>>>
> > >>>> e.g.
> > >>>>
> > >>>> F2FS_OPTION(sbi).fs_mode = FS_MODE_FRAGMENT_FIXED_BLK
> > >>>> fragment_chunk_size = 384
> > >>>> fragment_hole_size = 384
> > >>>>
> > >>>> When creating hole:
> > >>>>
> > >>>> - f2fs_allocate_data_block
> > >>>>     - __refresh_next_blkoff
> > >>>>       chunk locates in [0, 383] of current segment
> > >>>>       seg->next_blkoff = 384
> > >>>>       sbi->fragment_remained_chunk = 0
> > >>>>       then we will reset sbi->fragment_remained_chunk to 384
> > >>>>       and move seg->next_blkoff forward to 768 (384 + 384)
> > >>>>     - __has_curseg_space() returns false
> > >>>>     - allocate_segment() allocates new current segment
> > >>>>
> > >>>> So, for such case that hole may cross two segments, hole size may be truncated
> > >>>> to left size of previous segment.
> > >>>
> > >>> First, sbi->fragment_remained_chunk should be seg->fragment_remained_chunk.
> > >>
> > >> Oh, correct.
> > >>
> > >>> I understand what you mean, so you mean we need to take the leftover
> > >>> "hole" size over to the next segment?
> > >>> In the example, the leftover hole size will be (384 - (512-384)). Do
> > >>> you want to take this over to the next segment?
> > >>
> > >> Yes, the left 256 block-sized hole should be created before next chunk
> > >> in next opened segment.
> > >>
> > >
> > > Chao,
> > >
> > > Do you have any decent idea to pass the left hole size to the next
> > > segment which will be allocated?
> >
> > Daeho,
> >
> > I guess we can record left hole size in seg->fragment_remained_hole.
> >
>
> I understand we need a new fragment_remained_hole variable in segment structure.
> But, I mean.. How can we pass over the left hole size from the
> previous segment to the next segment?
>

I mean we don't know which segment will be the next segment, do we?

> Thanks,
>
> > Thanks,
> >
> > >
> > > Thanks,
> > >
> > >> Thanks,
> > >>
> > >>>