On Fri, Jan 22, 2021 at 09:14:08PM +0800, Gao Xiang wrote:

Hi Weiwen,

On Tue, Jan 19, 2021 at 01:49:51PM +0800, Hu Weiwen wrote:

...

 	bb = NULL;

-	list_for_each_entry(cur, &blkh.list, list) {
-		unsigned int used_before, used;
+	if (!used0 || alignsize == EROFS_BLKSIZ)
+		goto alloc;
+
+	/* try to find a most-fit mapped buffer block first */
+	used_before = EROFS_BLKSIZ -
+		round_up(size + required_ext + inline_ext, alignsize);

Honestly, after seen above I feel I'm not good at math now
since I smell somewhat strange of this, apart from the pending
patch you raised [1], the algebra is

/* since all buffers should be aligned with alignsize */
erofs_off_t alignedoffset = roundup(used_before, alignsize);

and (alignedoffset + size + required_ext + inline_ext <= EROFS_BLKSIZ)

and why it can be equal to
used_before = EROFS_BLKSIZ - round_up(size + required_ext + inline_ext, alignsize);

Could you explain this in detail if possible? for example,
size = 3
inline_ext = 62
alignsize = 32

so 4096 - roundup(3 + 62, 32) = 4096 - 96 = 4000
but, the real used_before can be even started at 4032, since
alignedoffset = roundup(4032, 32) = 4032
4032 + 62 = 4094 <= EROFS_BLKSIZ.

Am I stll missing something?

Oh, the example itself is wrong, yet I still feel no good at
this formula, e.g I'm not sure if it works for alignsize which
cannot be divided by EROFS_BLKSIZ (although currently alignsize =
4 or 32)

Thanks,
Gao Xiang

We can divide several parts of data in EROFS_BLKSIZ  as follows:

Use alignsize to represent these data:

1) alignsize * num_x = used_before + alignedoffset
2) alignsize * num_y = size + required_ext + inline_ext + tail_data
3) alignsize * num_z = EROFS_BLKSIZ

So we can get,
4) num_x + num_y = num_z

If we use
used_before = EROFS_BLKSIZ - round_up(size + required_ext + inline_ext, alignsize); 
here, num_y should be an integer.

Consider the following two situations: 

1) If EROFS_BLKSIZ can be divisible by alignsize, num_z is an integer. so num_x is an integer.
   The following formula can be satisfied:
   erofs_off_t alignedoffset = roundup(used_before, alignsize);

2）If EROFS_BLKSIZ can't be divisible by alignsize, num_z isn't an integer and num_x won't be an integer.
   The formula can't be satisfied.

So I think it should be
used_before = round_down(EROFS_BLKSIZ - size-required_ext - inline_ext , alignsize);
here.

Sorry for my poor english and figure. . .

Thanks,
Jianan

IMO, I don't want too hard on such math, I'd like to just use
used_before = EROFS_BLKSIZ - (size + required_ext + inline_ext);
and simply skip the bb if __erofs_battach is fail (as I said before,
the internal __erofs_battach can be changed, and I don't want to
imply that always succeed.)

If you also agree that, I'll send out a revised version along
with a cleanup patch to clean up erofs_balloc() as well, which
is more complicated than before.

[1] https://lore.kernel.org/r/20210121162606.8168-1-sehuww@mail.scut.edu.cn/

Thanks,
Gao Xiang