Re: [RFC PATCH] f2fs: restructure f2fs page.private layout

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 26, 2021 at 06:09:08PM +0800, Chao Yu wrote:
> Restruct f2fs page private layout for below reasons:
> 
> There are some cases that f2fs wants to set a flag in a page to
> indicate a specified status of page:
> a) page is in transaction list for atomic write
> b) page contains dummy data for aligned write
> c) page is migrating for GC
> d) page contains inline data for inline inode flush
> e) page is verified in merkle tree for fsverity

hm, why do you need to record that?  I would have thought that if a file
is marked as being protected by the merkle tree then any pages read in
would be !uptodate until the merkle tree verification had happened.

> f) page is dirty and has filesystem/inode reference count for writeback
> g) page is temporary and has decompress io context reference for compression
> 
> There are existed places in page structure we can use to store
> f2fs private status/data:
> - page.flags: PG_checked, PG_private
> - page.private
> 
> However it was a mess when we using them, which may cause potential
> confliction:
> 		page.private	PG_private	PG_checked
> a)		-1		set
> b)		-2
> c), d), e)					set
> f)		0		set
> g)		pointer		set
> 
> The other problem is page.flags has no free slot, if we can avoid set
> zero to page.private and set PG_private flag, then we use non-zero value
> to indicate PG_private status, so that we may have chance to reclaim
> PG_private slot for other usage. [1]
> 
> So in this patch let's restructure f2fs' page.private as below:
> 
> Layout A: lowest bit should be 1
> | bit0 = 1 | bit1 | bit2 | ... | bit MAX | private data .... |
>  bit 0	PAGE_PRIVATE_NOT_POINTER
>  bit 1	PAGE_PRIVATE_ATOMIC_WRITE
>  bit 2	PAGE_PRIVATE_DUMMY_WRITE
>  bit 3	PAGE_PRIVATE_ONGOING_MIGRATION
>  bit 4	PAGE_PRIVATE_INLINE_INODE
>  bit 5	PAGE_PRIVATE_REF_RESOURCE
>  bit 6-	f2fs private data
> 
> Layout B: lowest bit should be 0
>  page.private is a wrapped pointer.
> 
> After the change:
> 		page.private	PG_private	PG_checked
> a)		11		set
> b)		101
> c)		1001
> d)		10001
> e)						set
> f)		100001		set
> g)		pointer		set

Mmm ... this isn't enough to let us remove PG_private.  We'd need PG_private
to be set for b,c,d as well.

> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
> index 817d0bcb5c67..e393a67a023c 100644
> --- a/fs/f2fs/checkpoint.c
> +++ b/fs/f2fs/checkpoint.c
> @@ -444,7 +444,11 @@ static int f2fs_set_meta_page_dirty(struct page *page)
>  	if (!PageDirty(page)) {
>  		__set_page_dirty_nobuffers(page);
>  		inc_page_count(F2FS_P_SB(page), F2FS_DIRTY_META);
> -		f2fs_set_page_private(page, 0);
> +		set_page_private_reference(page);
> +		if (!PagePrivate(page)) {
> +			SetPagePrivate(page);
> +			get_page(page);
> +		}

I'm not a big fan of this pattern (which seems to be repeated quite often)
I think it should be buried within set_page_private_reference().  Also,
are the states abcdf all mutually exclusive, or can a page be in states
(eg) b and d at the same time?

> -		if (IS_DUMMY_WRITTEN_PAGE(page)) {
> -			set_page_private(page, (unsigned long)NULL);
> +		if (page_private_dummy(page)) {
> +			clear_page_private_dummy(page);
>  			ClearPagePrivate(page);

I think the ClearPagePrivate should be buried in the page_private_dummy()
macro too ... and shouldn't there be a put_page() for this too?