linux-kernel - Re: [PATCH V2 4/8] mm/highmem: Add VM_BUG_ON() to mem*

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Date:   Wed, 10 Feb 2021 06:57:30 +0000
From:   Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>
To:     "ira.weiny@...el.com" <ira.weiny@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Sterba <dsterba@...e.cz>
CC:     "hch@...radead.org" <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "clm@...com" <clm@...com>,
        "josef@...icpanda.com" <josef@...icpanda.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH V2 4/8] mm/highmem: Add VM_BUG_ON() to mem*_page() calls

On 2/9/21 22:25, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
>
> Add VM_BUG_ON bounds checks to ensure the newly lifted and created page
> memory operations do not result in corrupted data in neighbor pages and
> to make them consistent with zero_user().[1][2]
>
I did not understand this, in my tree :-

zero_user()
 -> zero_user_segments()

which uses BUG_ON(), the commit log says add VM_BUG_ON(), isn't that
inconsistent withwhat is there in zero_user_segments() which uses BUG_ON() ?

Also, this patch uses BUG_ON() which doesn't match the commit log that says
ADD VM_BUG_ON(),

Did I interpret the commit log wrong ?

[1]
 void zero_user_segments(struct page *page, unsigned start1, unsigned end1,
365                 unsigned start2, unsigned end2)
366 {
367         unsigned int
i;                                                                           

368
369         BUG_ON(end1 > page_size(page) || end2 > page_size(page));
370
371         for (i = 0; i < compound_nr(page); i++) {
372                 void *kaddr = NULL;
373 
374                 if (start1 < PAGE_SIZE || start2 < PAGE_SIZE)
375                         kaddr = kmap_atomic(page + i);
376
377                 if (start1 >= PAGE_SIZE) {
378                         start1 -= PAGE_SIZE;
379                         end1 -= PAGE_SIZE;
380                 } else {
381                         unsigned this_end = min_t(unsigned, end1,
PAGE_SIZE);
382        
383                         if (end1 > start1)
384                                 memset(kaddr + start1, 0, this_end -
start1);
385                         end1 -= this_end;
386                         start1 = 0;
387                 }
388
389                 if (start2 >= PAGE_SIZE) {
390                         start2 -= PAGE_SIZE;
391                         end2 -= PAGE_SIZE;
392                 } else {
393                         unsigned this_end = min_t(unsigned, end2,
PAGE_SIZE);
394 
395                         if (end2 > start2)
396                                 memset(kaddr + start2, 0, this_end -
start2);
397                         end2 -= this_end;
398                         start2 = 0;
399                 }
400        
401                 if (kaddr) {
402                         kunmap_atomic(kaddr);
403                         flush_dcache_page(page + i);
404                 }
405        
406                 if (!end1 && !end2)
407                         break;
408         }
409        
410         BUG_ON((start1 | start2 | end1 | end2) != 0);
411 }
412 EXPORT_SYMBOL(zero_user_segments);