On Wed, Apr 04, 2018 at 07:45:53AM +0200, Michal Hocko wrote:
On Wed 04-04-18 09:33:57, Wei Yang wrote:
On Tue, Apr 03, 2018 at 01:30:41PM +0200, Michal Hocko wrote:
On Mon 02-04-18 09:50:26, Wei Yang wrote:
On Fri, Mar 30, 2018 at 01:57:27PM -0700, Andrew Morton wrote:
On Fri, 30 Mar 2018 11:30:55 +0800 Wei Yang richard.weiyang@gmail.com wrote:
memblock_search_pfn_nid() returns the nid and the [start|end]_pfn of the memory region where pfn sits in. While the calculation of start_pfn has potential issue when the regions base is not page aligned.
For example, we assume PAGE_SHIFT is 12 and base is 0x1234. Current implementation would return 1 while this is not correct.
Why is this not correct? The caller might want the pfn of the page which covers the base?
Hmm... the only caller of memblock_search_pfn_nid() is __early_pfn_to_nid(), which returns the nid of a pfn and save the [start_pfn, end_pfn] with in the same memory region to a cache. So this looks not a good practice to store un-exact pfn in the cache.
This patch fixes this by using PFN_UP().
The original commit is commit e76b63f80d93 ("memblock, numa: binary search node id") and merged in v3.12.
Signed-off-by: Wei Yang richard.weiyang@gmail.com Cc: 3.12+ stable@vger.kernel.org
Please fully describe the runtime effects of a bug when fixing that bug. This description doesn't give enough justification for merging the patch into mainline, let alone -stable.
Since PFN_UP() and PFN_DOWN() differs when the address is not page aligned, in theory we may have two situations like below.
Have you ever seen a HW that would report page unaligned memory ranges? Is this even possible?
No, so we don't need to handle this case?
Memblock code is subtle enough to not touch it for something that doesn't really exist. We do have some alignment assumptions all over so if we have weird configurations with page non-aligned regions of memory then we should do the rounding when this is detected rathert than spread them all over random places.
Well, I got you concern and I agree to have some assumption to make the code easy to read or write.
But, my point is current code doesn't keep this assumption always. For example, __next_mem_pfn_range() and memblock_search_pfn_nid() treat the pfn range differently. The inconsistency may confuse audience and harm the kernel in the long turn.
To not spread this calculation over random places, how about introducing a helper function memblock_pfn_range() to consolidate the place and help audience in the future.
For example, https://patchwork.kernel.org/patch/10323845/ tries to optimize the page initialization which rely on the memblock pfn range. If we could consolidate the calculation in one place, would it be more convenient to maintain the kernel?
-- Michal Hocko SUSE Labs