linux-kernel - Re: [PATCH v8 06/10] iomap: fix iomap_dio

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Message-ID: <20240702101556.jdi5anyr3v5zngnv@quentin>
Date: Tue, 2 Jul 2024 10:15:56 +0000
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Christoph Hellwig <hch@....de>
Cc: david@...morbit.com, willy@...radead.org, chandan.babu@...cle.com,
	djwong@...nel.org, brauner@...nel.org, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, yang@...amperecomputing.com,
	linux-mm@...ck.org, john.g.garry@...cle.com,
	linux-fsdevel@...r.kernel.org, hare@...e.de, p.raghav@...sung.com,
	mcgrof@...nel.org, gost.dev@...sung.com, cl@...amperecomputing.com,
	linux-xfs@...r.kernel.org, Zi Yan <zi.yan@...t.com>
Subject: Re: [PATCH v8 06/10] iomap: fix iomap_dio_zero() for fs bs > system
 page size

> > +fs_initcall(iomap_pagecache_init);
> 
> s/iomap_pagecache_init/iomap_buffered_init/
> 
> We don't use pagecache naming anywhere else in the file.

Got it.
> 
> > +/*
> > + * Used for sub block zeroing in iomap_dio_zero()
> > + */
> > +#define ZERO_PAGE_64K_SIZE (65536)
> 
> just use SZ_64K
> 
> > +#define ZERO_PAGE_64K_ORDER (get_order(ZERO_PAGE_64K_SIZE))
> 
> No really point in having this.

Hmm, I used it twice, hence the define. But if we decide to get rid of
set_memory_ro(), then this does not make sense.

> 
> > +static struct page *zero_page_64k;
> 
> This should be a folio.  Encoding the size in the name is also really
> weird and just creates churn when we have to increase it.

Willy suggested we could use raw pages as we don't need the metadata
from using a folio. [0]

> 
> 
> > +	/*
> > +	 * Max block size supported is 64k
> > +	 */
> > +	WARN_ON_ONCE(len > ZERO_PAGE_64K_SIZE);
> 
> 
> A WARN_ON without actually erroring out here is highly dangerous. 

I agree but I think we decided that we are safe with 64k for now as fs 
that uses iomap will not have a block size > 64k. 

But this function needs some changes when we decide to go beyond 64k
by returning error instead of not returning anything. 
Until then WARN_ON_ONCE would be a good stop gap for people developing
the feature to go beyond 64k block size[1]. 

> 
> > +
> >  	bio = iomap_dio_alloc_bio(iter, dio, 1, REQ_OP_WRITE | REQ_SYNC | REQ_IDLE);
> 
> Overly long line here.
> 

Not a part of my change, so I didn't bother reformatting it. :)

> > +
> > +static int __init iomap_dio_init(void)
> > +{
> > +	zero_page_64k = alloc_pages(GFP_KERNEL | __GFP_ZERO,
> > +				    ZERO_PAGE_64K_ORDER);
> 
> > +
> > +	if (!zero_page_64k)
> > +		return -ENOMEM;
> > +
> > +	set_memory_ro((unsigned long)page_address(zero_page_64k),
> > +		      1U << ZERO_PAGE_64K_ORDER);
> 
> What's the point of the set_memory_ro here?  Yes, we won't write to
> it, but it's hardly an attack vector and fragments the direct map.

That is a good point. Darrick suggested why not add a ro tag as we don't
write to it but I did not know the consequence of direct map
fragmentation when this is added. So probably there is no value calling
set_memory_ro here.

--
Pankaj

[0] https://lore.kernel.org/linux-fsdevel/ZkT46AsZ3WghOArL@casper.infradead.org/
[1] I spent a lot of time banging my head why I was getting FS corruption
when I was doing direct io in XFS while adding LBS support before I found
the PAGE_SIZE assumption here.