linux-kernel - Re: [PATCH v3 1/2] kho: add support for preserving vmalloc allocations

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Message-ID: <aMgey9qEC_XUejXm@kernel.org>
Date: Mon, 15 Sep 2025 17:12:27 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Pratyush Yadav <me@...avpratyush.com>
Cc: Pratyush Yadav <pratyush@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Graf <graf@...zon.com>, Baoquan He <bhe@...hat.com>,
	Changyuan Lyu <changyuanl@...gle.com>, Chris Li <chrisl@...nel.org>,
	Jason Gunthorpe <jgg@...dia.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	kexec@...ts.infradead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] kho: add support for preserving vmalloc
 allocations

On Tue, Sep 09, 2025 at 04:33:27PM +0200, Pratyush Yadav wrote:
> Hi Mike,
> 
> Couple more thoughts.
> 
> On Mon, Sep 08 2025, Pratyush Yadav wrote:
> > On Mon, Sep 08 2025, Mike Rapoport wrote:
> >> +
> >> +	while (chunk) {
> >> +		struct page *page;
> >> +
> >> +		for (int i = 0; i < chunk->hdr.num_elms; i++) {
> >> +			phys_addr_t phys = chunk->phys[i];
> >> +
> >> +			for (int j = 0; j < (1 << order); j++) {
> >> +				page = phys_to_page(phys);
> >> +				kho_restore_page(page, 0);
> >> +				pages[idx++] = page;
> >
> > This can buffer-overflow if the previous kernel was buggy and added too
> > many pages. Perhaps keep check for this?
> 
> Thinking about this a bit more, I think this should check that we found
> _exactly_ chunk->hdr.total_pages pages, and should error out otherwise.
> If too few are found, pages array will contain bogus data, if too many,
> buffer overflow.

Sure, I can add the checks, but it feels superfluous to me.
 
> Also, I am not a fan of using kho_restore_page() directly. I think the
> vmalloc preservation is a layer above core KHO, and it should use the
> public KHO APIs. It really doesn't need to poke into internal APIs. If
> any of the public APIs are insufficient, we should add new ones.
> 
> I don't suppose I'd insist on it, but something to consider since you
> are likely going to do another revision anyway.

I think vmalloc is as basic as folio. At some point we probably converge to 
kho_preserve(void *) that will choose the right internal handler. like
folio, vmalloc, kmalloc etc.

> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.