Re: [syzbot] [mm?] kernel BUG in sg_init_one

[Ira Weiny <ira.weiny@intel.com>]

Yosry Ahmed wrote:
> On Wed, Feb 12, 2025 at 09:20:24AM -0800, Yosry Ahmed wrote:
> > On Mon, Mar 18, 2024 at 2:03 PM Barry Song <21cnbao@gmail.com> wrote:
> > >
> > > On Tue, Mar 19, 2024 at 9:52 AM syzbot
> > > <syzbot+adbc983a1588b7805de3@syzkaller.appspotmail.com> wrote:
> > > >
> > > > Hello,
> > > >
> > > > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > > > WARNING in __kmap_to_page
> > > >
> > > > ------------[ cut here ]------------
> > > > WARNING: CPU: 0 PID: 3529 at mm/highmem.c:167 __kmap_to_page+0x100/0x194 mm/highmem.c:167
> > > > Modules linked in:
> > >
> > > + Ira
> > >
> > > Hi Ira,
> > >
> > > I noticed this warning is coming from commit ef6e06b2ef87077.
> > >
> > > you have a commit message like
> > > "    Because it is intended to remove kmap_to_page() add a warn on once to
> > >     the kmap checks to flag potential issues early.
> > > "
> > >
> > > Do we have a replacement for kmap_to_page()? The background is that we
> > > want to pass a highmem buffer to sg_set_page() but we only know its virt
> > > address.
> > 
> > I am reviving this thread because new zsmalloc changes will make
> > mappings sleepable, which will allow zswap to drop the memcpy() in
> > zswap_decompress() -- except for the !virt_addr_valid() case. We can
> > get rid of that too if we can use kmap_tp_page() in the scatterlist
> > code.
> > 
> > Ira, could you please answer Barry's question above about
> > kmap_to_page()? It has been a year and kmap_to_page() is still around.
> 
> (Trying again with Ira as the main recepient just in case)
> 
> Ira, could you please help us out here? :)

Apologies,

No there is no alternative to kmap_to_page().  The work I was doing has
stalled out and I really don't know if it will resume.

There are a few folks, like me, who would like to remove highmem but every
time the subject comes up someone speaks up about a rare (mostly embedded)
platform which really needs it.  So I don't see it going away soon.

Removing the warning could be justified by saying that highmem removal can
be done completely within the kmap calls and only when that has been
completed can these calls go away.  But generally kmap_to_page() is not a
popular call and it might be seen as a step backwards by some.

For example:
	https://lore.kernel.org/linux-mm/20221216070621.GA24832@lst.de/

The patch with the warnings was a stop gap to ensure current users did not
break.

Do you have evidence that the extra memcpy is bad enough performance that
you could justify using kmap_to_page?

Ira

> 
> > 
> > Thanks.
>