Re: [PATCH v2 00/21] netfs: Keep track of folios in a segmented bio_vec[] chain

[David Laight]

On Mon, 18 May 2026 23:29:32 +0100
David Howells <dhowells@xxxxxxxxxx> wrote:

> Hi Christian,
> 
> Could you add these patches to the VFS tree for next?
> 
> The patches get rid of folio_queue, rolling_buffer and ITER_FOLIOQ,
> replacing the folio queue construct used to manage buffers in netfslib with
> one based around a segmented chain of bio_vec arrays instead.  There are
> three main aims here:
> 
>  (1) The kernel file I/O subsystem seems to be moving towards consolidating
>      on the use of bio_vec arrays, so embrace this by moving netfslib to
>      keep track of its buffers for buffered I/O in bio_vec[] form.
> 
>  (2) Netfslib already uses a bio_vec[] to handle unbuffered/DIO, so the
>      number of different buffering schemes used can be reduced to just a
>      single one.
> 
>  (3) Always send an entire filesystem RPC request message to a TCP socket
>      with single kernel_sendmsg() call as this is faster, more efficient
>      and doesn't require the use of corking as it puts the entire
>      transmission loop inside of a single tcp_sendmsg().
> 
> For the replacement of folio_queue, a segmented chain of bio_vec arrays
> rather than a single monolithic array is provided:
> 
> 	struct bvecq {
> 		struct bvecq		*next;
> 		struct bvecq		*prev;
> 		unsigned long long	fpos;
> 		refcount_t		ref;
> 		u32			priv;
> 		u16			nr_segs;
> 		u16			max_segs;
> 		enum bvecq_mem		mem_type:2;
> 		bool			inline_bv:1;
> 		bool			discontig:1;

There doesn't seem to be any point using bitfields.
There is a massive hole here anyway.

> 		struct bio_vec		*bv;
> 		struct bio_vec		__bv[];
> 	};
> 
> The fields are:
> 
>  (1) next, prev - Link segments together in a list.  I want this to be
>      NULL-terminated linear rather than circular to make it possible to
>      arbitrarily glue bits on the front.

Do you ever need to follow the list backwards?
If not making prev point to the pointer to the entry (probably a tailq?)
makes the logic simpler (and safer) because you can remove an item without
knowing whether it is the head or which list it is on.

> 
>  (2) fpos, discontig - Note the current file position of the first byte of
>      the segment; all the bio_vecs in ->bv[] must be contiguous in the file
>      space.  The fpos can be used to find the folio by file position rather
>      then from the info in the bio_vec.

Should fpos be off_t (or u64) rather than 'long long' (they are all the
same underlying type).

>      If there's a discontiguity, this should break over into a new bvecq
>      segment with the discontig flag set (though this is redundant if you
>      keep track of the file position).  Note that the beginning and end
>      file positions in a segment need not be aligned to any filesystem
>      block size.

At this point you lose me :-)

-- David