linux-kernel - Re: [PATCH] fs/buffer: use min folio order to calculate upper limit in __getblk

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Message-ID: <lv3zoqm3uuzfqskcr734btb3hgqy67ddmd4ik2vidl3y3qv2hj@2zb34igia4o5>
Date: Wed, 18 Jun 2025 21:50:56 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Jan Kara <jack@...e.cz>
Cc: Pankaj Raghav <p.raghav@...sung.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, mcgrof@...nel.org, Christian Brauner <brauner@...nel.org>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, gost.dev@...sung.com
Subject: Re: [PATCH] fs/buffer: use min folio order to calculate upper limit
 in __getblk_slow()

> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 8cf4a1dc481e..98f90da69a0a 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -1121,10 +1121,11 @@ __getblk_slow(struct block_device *bdev, sector_t block,
> >  	     unsigned size, gfp_t gfp)
> >  {
> >  	bool blocking = gfpflags_allow_blocking(gfp);
> > +	int blocklog = PAGE_SHIFT + mapping_min_folio_order(bdev->bd_mapping);
> >  
> >  	/* Size must be multiple of hard sectorsize */
> > -	if (unlikely(size & (bdev_logical_block_size(bdev)-1) ||
> > -			(size < 512 || size > PAGE_SIZE))) {
> > +	if (unlikely(size & (bdev_logical_block_size(bdev) - 1) ||
> > +		     (size < 512 || size > (1U << blocklog)))) {
> 
> So this doesn't quite make sense to me.  Shouldn't it be capped from above
> by PAGE_SIZE << mapping_max_folio_order(bdev->bd_mapping)?

This __getblk_slow() function is used to read a block from a block
device and fill the page cache along with creating buffer heads.

I think the reason we have this check is to make sure the size, which is
block size is within the limits from 512 (SECTOR_SIZE) to upper limit on block size.

That upper limit on block size was PAGE_SIZE before the lbs support in 
block devices, but now the upper limit of block size is mapping_min_folio_order.
We set that in set_blocksize(). So a single block cannot be bigger than
(PAGE_SIZE << mapping_min_folio_order).

I hope that makes sense.

-- 
Pankaj Raghav