Re: [Qemu-devel] [PATCH v2 1/3] linux-user/mmap.c: Set prot page flags for the correct region in mmap_frag()

[Chen Gang <chengang@emindsoft.com.cn>]

On 2016年01月26日 18:26, Peter Maydell wrote:
> On 26 January 2016 at 10:19, Chen Gang <chengang@emindsoft.com.cn> wrote:
>> When I run WeChat.exe with i386 wine with qemu-i386 under sw_64 arch.
>>
>>  - The related command:
>>
>>    "./i386-linux-user/qemu-i386 -strace -L /upstream/i386_wine /upstream/i386_wine/usr/local/bin/wine "C:\\Program Files\\Tencent\\WeChat\\WeChat.exe" > ana/try/info-strace.log 2>&1"
>>
>>  - The related output (no any munmap, 135168 = 128KB + 4KB):
>>
>>    4600 mmap2(0x00340000,135168,PROT_READ,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0) = 0x00340000
>>    4600 mmap2(0x00340000,135168,PROT_READ,MAP_SHARED|MAP_FIXED,8,0) = 0x00340000
>>    4600 rt_sigprocmask(SIG_SETMASK,0x0033f574,NULL) = 0
>>    4600 rt_sigprocmask(SIG_BLOCK,0x7bced7e0,0x0033f5d0) = 0
>>    4600 write(3,0x33f6cc,64) = 64
>>    4600 read(4,0x33f6cc,64) = 1
>>    4600 rt_sigprocmask(SIG_SETMASK,0x0033f5d0,NULL) = 0
>>    4600 close(8) = 0
>>    4600 rt_sigprocmask(SIG_BLOCK,0x7bced7e0,0x0033f674) = 0
>>    4600 mprotect(0x00160000,65536,PROT_READ|PROT_WRITE) = 0
>>    4600 rt_sigprocmask(SIG_SETMASK,0x0033f674,NULL) = 0
>>    4600 rt_sigprocmask(SIG_BLOCK,0x7bced7e0,0x0033f990) = 0
>>    4600 mmap2(0x00340000,135168,PROT_NONE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED|MAP_NORESERVE,-1,0) = 0x00340000
>>
>> wine often does like above, map the same position multiple times.
> 
> That output seems to show all the mmap calls working fine, though.
>

OK, thanks.
 
>>
>> For the case I provide above, it can call mmap_frag() twice for the same
>> host page.
> 
> For the same single call to target_mmap() ? What is the code flow
> within QEMU that causes this?
> 

Within one single call to target_mmap(), it should be OK.

But multiple call to target_mmap(), may call mmap_frag() multiple times
for the same host page (also for the same target page). In our case:

 - 4600 mmap2(0x00340000,135168,PROT_READ,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0) = 0x00340000

   It will call mmap_frag() with start address 0x00340000 + 128KB, and
   set the target page with PAGE_VALID. But left the half below host
   page without PAGE_VALID.

 - 4600 mmap2(0x00340000,135168,PROT_READ,MAP_SHARED|MAP_FIXED,8,0) = 0x00340000

   It will call mmap_frag() with start address 0x00340000 + 128KB, and
   check the half below host page which has no PAGE_VALID, then "prot1
   == 0", mmap_frag() thinks "no page was there, so we allocate one".

 - But in fact, the first mmap_frag() has already allocated one page at
   0x00340000 + 128KB.

Thanks.
-- 
Chen Gang (陈刚)

Open, share, and attitude like air, water, and life which God blessed