Re: [PATCH] arm: flush: don't abuse pfn_valid() to check if pfn is in RAM

[Yongqiang Liu <liuyongqiang13@huawei.com>]

Very appreciate it for extra explanation. Notice that commit 024591f9a6e0

("arm: ioremap: don't abuse pfn_valid() to check if pfn is in RAM") use

memblock_is_map_memory() instead of pfn_valid() to check if a PFN is in

RAM or not, so I wrote the patch to solve this case.  Otherwise, when we

use pageblock align(4M) address of memory or uio, like :

      node   0: [mem 0x00000000c0c00000-0x00000000cc8fffff]
      node   0: [mem 0x00000000d0000000-0x00000000da1fffff]

or uio address set like:

    0xc0400000, 0x100000

the pfn_valid will return false as memblock_is_map_memory.

在 2024/2/1 5:20, Robin Murphy 写道:
> On 2024-01-31 7:00 pm, Russell King (Oracle) wrote:
>> On Wed, Jan 31, 2024 at 06:39:31PM +0000, Robin Murphy wrote:
>>> On 31/01/2024 12:59 pm, Yongqiang Liu wrote:
>>>> @@ -292,7 +293,7 @@ void __sync_icache_dcache(pte_t pteval)
>>>>            /* only flush non-aliasing VIPT caches for exec mappings */
>>>>            return;
>>>>        pfn = pte_pfn(pteval);
>>>> -    if (!pfn_valid(pfn))
>>>> +    if (!memblock_is_map_memory(PFN_PHYS(pfn)))
>>>>            return;
>>>>        folio = page_folio(pfn_to_page(pfn));
>>>
>>> Hmm, it's a bit odd in context, since pfn_valid() obviously pairs 
>>> with this
>>> pfn_to_page(), whereas it's not necessarily clear that
>>> memblock_is_map_memory() implies pfn_valid().
>>>
>>> However, in this case we're starting from a PTE - rather than going 
>>> off to
>>> do a slow scan of memblock to determine whether a round-trip through
>>> page_address() is going to give back a mapped VA, can we not trivially
>>> identify that from whether the PTE itself is valid?
>>
>> Depends what you mean by "valid". If you're referring to pte_valid()
>> and L_PTE_VALID then no.
>>
>> On 32-bit non-LPAE, the valid bit is the same as the present bit, and
>> needs to be set for the PTE to not fault. Any PTE that is mapping
>> something will be "valid" whether it is memory or not, whether it is
>> backed by a page or not.
>>
>> pfn_valid() should be telling us whether the PFN is suitable to be
>> passed to pfn_to_page(), and if we have a situation where pfn_valid()
>> returns true, but pfn_to_page() returns an invalid page, then that in
>> itself is a bug that needs to be fixed and probably has far reaching
>> implications for the stability of the kernel.
>
> Right, the problem here seems to be the opposite one, wherein we *do* 
> often have a valid struct page for an address which is reserved and 
> thus not mapped by the kernel, but seemingly we then take it down a 
> path which assumes anything !PageHighmem() is lowmem and dereferences 
> page_address() without looking.
>
> However I realise I should have looked closer at the caller, and my 
> idea is futile since the PTE here is for a userspace mapping, not a 
> kernel VA, and is already pte_valid_user() && !pte_special(). Plus the 
> fact that the stack trace indicates an mmap() path suggests it most 
> likely is a legitimate mapping of some no-map carveout or MMIO region. 
> Oh well. My first point still stands, though - I think at least a 
> comment to clarify that assumption would be warranted.
>
> Thanks,
> Robin.
> .