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Date: Tue, 16 Jun 2009 19:35:24 -0700
Message-ID: <6934efce0906161935x65c2a31br4bf1d35493e7b77c@mail.gmail.com>
Subject: Re: [PATCH 13/14] Pramfs: Write protection
From: Jared Hulbert <jaredeh@gmail.com>
To: Marco <marco.stornelli@gmail.com>
Cc: Linux FS Devel <linux-fsdevel@vger.kernel.org>,
       Linux Embedded <linux-embedded@vger.kernel.org>,
       Linux Kernel <linux-kernel@vger.kernel.org>,
       Daniel Walker <dwalker@soe.ucsc.edu>
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Lines: 77

> +/* init_mm.page_table_lock must be held before calling! */
> +static void pram_page_writeable(unsigned long addr, int rw)
> +{
> + ? ? ? pgd_t *pgdp;
> + ? ? ? pud_t *pudp;
> + ? ? ? pmd_t *pmdp;
> + ? ? ? pte_t *ptep;
> +
> + ? ? ? pgdp = pgd_offset_k(addr);
> + ? ? ? if (!pgd_none(*pgdp)) {
> + ? ? ? ? ? ? ? pudp = pud_offset(pgdp, addr);
> + ? ? ? ? ? ? ? if (!pud_none(*pudp)) {
> + ? ? ? ? ? ? ? ? ? ? ? pmdp = pmd_offset(pudp, addr);
> + ? ? ? ? ? ? ? ? ? ? ? if (!pmd_none(*pmdp)) {
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? pte_t pte;
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ptep = pte_offset_kernel(pmdp, addr);
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? pte = *ptep;
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? if (pte_present(pte)) {
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? pte = rw ? pte_mkwrite(pte) :
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? pte_wrprotect(pte);
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? set_pte(ptep, pte);
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? }
> + ? ? ? ? ? ? ? ? ? ? ? }
> + ? ? ? ? ? ? ? }
> + ? ? ? }
> +}

Wow.  Don't we want to do this pte walking in mm/ someplace?

Do you really intend to protect just the PTE in question rather than
the entire physical page, regardless of which PTE is talking to it?
Maybe I'm missing something.

> +/* init_mm.page_table_lock must be held before calling! */
> +void pram_writeable(void *vaddr, unsigned long size, int rw)
> +{
> + ? ? ? unsigned long addr = (unsigned long)vaddr & PAGE_MASK;
> + ? ? ? unsigned long end = (unsigned long)vaddr + size;
> + ? ? ? unsigned long start = addr;
> +
> + ? ? ? do {
> + ? ? ? ? ? ? ? pram_page_writeable(addr, rw);
> + ? ? ? ? ? ? ? addr += PAGE_SIZE;
> + ? ? ? } while (addr && (addr < end));
> +
> +
> + ? ? ? /*
> + ? ? ? ?* NOTE: we will always flush just one page (one TLB
> + ? ? ? ?* entry) except possibly in one case: when a new
> + ? ? ? ?* filesystem is initialized at mount time, when pram_read_super
> + ? ? ? ?* calls pram_lock_range to make the super block, inode
> + ? ? ? ?* table, and bitmap writeable.
> + ? ? ? ?*/
> +#if defined(CONFIG_ARM) || defined(CONFIG_M68K) || defined(CONFIG_H8300) || \
> + ? ? ? defined(CONFIG_BLACKFIN)
> + ? ? ? /*
> + ? ? ? ?* FIXME: so far only these archs have flush_tlb_kernel_page(),
> + ? ? ? ?* for the rest just use flush_tlb_kernel_range(). Not ideal
> + ? ? ? ?* to use _range() because many archs just flush the whole TLB.
> + ? ? ? ?*/
> + ? ? ? if (end <= start + PAGE_SIZE)
> + ? ? ? ? ? ? ? flush_tlb_kernel_page(start);
> + ? ? ? else
> +#endif
> + ? ? ? ? ? ? ? flush_tlb_kernel_range(start, end);
> +}

Why not just fix flush_tlb_range()?

If an arch has a flush_tlb_kernel_page() that works then it stands to
reason that the flush_tlb_kernel_range() shouldn't work with minimal
effort, no?
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