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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1758034330; bh=i8NtzmKVeK0CBqBApTYTm3iAf9ez5i62j0SbReGfb7c=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=t3jMv2cpq05jLr5mya8gnqY0LdWfZKH172NqGWZsUQV450OQ7MSecKK/BvQvPsF61 +wRfIMDMNu5N52JqAVHgKcJg+2qotenh8Q40u6DpJOiuUnYo86PyubV2WqMSegeBGj /7EIp+/ORtdmETQpkePK3TluVJdEAbAnGdEnNUEoujU0XWKotd4E5E3BzaT5jVeLRq q6m3409DAFi74aePUAzsstLsPnsK+X1OW8V6f3ZUuR0xqUr7IONdQTxe9jWgDC7QYY i4xftMwQ9c57QB47kUCFlPqzeB9DyhXXjJvbEhIrpy8e9Z0pEWN0Zw4oGK9g15NxcR PlII1l1WkPEiA== From: Pratyush Yadav To: Jason Gunthorpe Cc: Pratyush Yadav , Pratyush Yadav , Matthew Wilcox , Alexander Graf , Mike Rapoport , Changyuan Lyu , Andrew Morton , Baoquan He , Pasha Tatashin , Chris Li , Jason Miu , linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-mm@kvack.org Subject: Re: [PATCH] kho: make sure folio being restored is actually from KHO In-Reply-To: <20250916132759.GC1086830@nvidia.com> References: <20250910153443.95049-1-pratyush@kernel.org> <20250910155546.GB922064@nvidia.com> <20250916132759.GC1086830@nvidia.com> Date: Tue, 16 Sep 2025 16:52:06 +0200 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 4A235140002 X-Stat-Signature: uw6z4tibsx9ypoyhy9jpnajha31j9rah X-Rspam-User: X-HE-Tag: 1758034331-325986 X-HE-Meta: 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 18D4XvTP KThcXJSjmNMM+iU3dMc08aWFi3L6ZoIkE4Vtw9xOxH/CyyqVcUZvym9qUs6STSTGC2ItIzVaqcogMYZTawcVVX/aQcTxBgdxZOhgM98ryGA4y34a7uCCvt2l9pQXGoNHa0mmogz1LEXw2Mc00WxO38gBd0IaX4DRjJpHPY02dGHCAVrg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Sep 16 2025, Jason Gunthorpe wrote: > On Tue, Sep 16, 2025 at 03:20:51PM +0200, Pratyush Yadav wrote: >> >> >> @@ -210,16 +226,16 @@ static void kho_restore_page(struct page *page, unsigned int order) >> >> >> struct folio *kho_restore_folio(phys_addr_t phys) >> >> >> { >> >> >> struct page *page = pfn_to_online_page(PHYS_PFN(phys)); >> >> >> - unsigned long order; >> >> >> + union kho_page_info info; >> >> >> >> >> >> if (!page) >> >> >> return NULL; >> >> >> >> >> >> - order = page->private; >> >> >> - if (order > MAX_PAGE_ORDER) >> >> >> + info.page_private = page->private; >> >> >> + if (info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER) >> > >> > All the impossible checks shoudl be WARN_ON() >> >> The mental model I have is that the place that introduced the >> "impossible" situation should get the WARN(). So for an incorrect phys >> address (leading to magic mismatch) or incorrect order (say preserved >> big range using kho_preserve_phys() and restoring it using >> kho_restore_folio()), the caller is responsible so it should do the >> WARN(). Does that make sense? > > Callers should not pass illegal phys here, WARN at this point is > appropriate, and maybe under a debug #ifdef or something like that. > > This is to make it clear that "test and fail" is not an acceptable way > to use this API. Okay, makes sense. Will add. > >> Actually, on another look, this patch implicitly makes sure that >> unaligned phys always fails. This is because deserialize_bitmap() only >> sets the magic on the head page which is always aligned by the order. >> For any unaligned phys, the magic will not match. > > Makes sense, maybe a comment? Will do. > >> Makes sense. Do you suggest the magic and order checks to also be under >> KHO_DEBUG, or should they always be done? I am of the opinion that it >> makes sense to do them always, but I can also understand the argument >> for disabling them in production for better performance. > > I'm ambivalent :) > > For now I'd do it always and leave some debug optimization to a future > project. Sounds good. Will add them as always for now. -- Regards, Pratyush Yadav