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

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Message-ID: <qu2m6fw64ikk5dapckywr7tuyqjsyktvn4hnr7nl7py7p5dpbt@tzbvv6ivfvuu>
Date: Thu, 19 Jun 2025 12:13:33 +0200
From: Jan Kara <jack@...e.cz>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Jan Kara <jack@...e.cz>, 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()

On Wed 18-06-25 21:50:56, Pankaj Raghav (Samsung) wrote:
> > > 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).

Ah, right. Thanks for explanation. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

									Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR