linux-kernel - Re: [PATCH v2] fs/buffer: use min folio order to calculate upper limit in __getblk

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Message-ID: <tuf33rhiur3h42o27sjrgs4jxhstmul3gqjbh6sjikhv7z3ntt@ebfq6ojqisp3>
Date: Thu, 19 Jun 2025 15:57:40 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Pankaj Raghav <p.raghav@...sung.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 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 v2] fs/buffer: use min folio order to calculate upper
 limit in __getblk_slow()

On Thu, Jun 19, 2025 at 01:59:12PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 19, 2025 at 02:10:58PM +0200, Pankaj Raghav wrote:
> > +++ b/fs/buffer.c
> > @@ -1121,9 +1121,10 @@ __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);
> >  
> >  	if (unlikely(size & (bdev_logical_block_size(bdev) - 1) ||
> > -		     (size < 512 || size > PAGE_SIZE))) {
> > +		     (size < 512 || size > (1U << blocklog)))) {
> >  		printk(KERN_ERR "getblk(): invalid block size %d requested\n",
> >  					size);
> >  		printk(KERN_ERR "logical block size: %d\n",
> 
> Is this what we want though?  If ext4 wants to create an 8kB block size
> filesystem on top of a 512 byte sector size device, shouldn't it be

That will not be a problem because we set the min order of the FS on the
block device[1] from ext4[2] through set_blocksize() routine.

> allowed to?  So just drop the max:

But I do agree with dropping it because we have these checks all over the
place. So the question is: do we need it again in a low level function
such as __getblk_slow().

> 
>  	if (unlikely(size & (bdev_logical_block_size(bdev) - 1) ||
> -		     (size < 512 || size > PAGE_SIZE))) {
> +		     (size < 512)))) {
> 
> (also, surely logical_block_size is always at least 512, so do we really
> need this check at all?)

True!

Just the alignment check with logical block size should be enough.

--
Pankaj

[1] https://elixir.bootlin.com/linux/v6.16-rc2/source/block/bdev.c#L210
[2] https://elixir.bootlin.com/linux/v6.16-rc2/source/fs/ext4/super.c#L5110