Hi,Dave
zram is same with other swap device,but every swaped page will be compressed then saved to another memory address.
The process is same with the common swap device,non-swap just a normal user address,pgd and mmu will translate to phy address
please refer to below information:
crash> vm -p
PID: 1565 TASK: ffffffe1fce32d00 CPU: 7 COMMAND: "system_server"
MM PGD RSS TOTAL_VM
ffffffe264431c00 ffffffe1f54ad000 528472k 9780384k
VMA START END FLAGS FILE
ffffffe0ea401300 12c00000 12e00000 100073
VIRTUAL PHYSICAL
...
144fc000 SWAP: /dev/block/zram0 OFFSET: 236750
...
1738e000 SWAP: /dev/block/zram0 OFFSET: 73426
1738f000 21aa2c000
17390000 1c3308000
17391000 SWAP: /dev/block/zram0 OFFSET: 73431
17392000 19c162000
17393000 19c132000
17394000 SWAP: /dev/block/zram0 OFFSET: 234576
17395000 19c369000
17396000 20b35c000
17397000 18011e000
17398000 SWAP: /dev/block/zram0 OFFSET: 73433
17399000 1dc3d2000
1739a000 1bc59f000
1739b000 SWAP: /dev/block/zram0 OFFSET: 73437
crash> vtop -c 1565 144fc000
VIRTUAL PHYSICAL
144fc000 (not mapped)
PAGE DIRECTORY: ffffffe1f54ad000
PGD: ffffffe1f54ad000 => 1f54ab003
PMD: ffffffe1f54ab510 => 1f43b8003
PTE: ffffffe1f43b87e0 => 39cce00
PTE SWAP OFFSET
39cce00 /dev/block/zram0 236750
VMA START END FLAGS FILE
ffffffe148bafe40 144c0000 14540000 100073
SWAP: /dev/block/zram0 OFFSET: 236750
________________________________________
From: Dave Anderson <anderson@xxxxxxxxxx>
Sent: Wednesday, April 1, 2020 22:20
To: d hatayama
Cc: 赵乾利; Discussion list for crash utility usage, maintenance and development
Subject: [External Mail]Re: zram decompress support for gcore/crash-utility
----- Original Message -----
...
> >
> > As far as the gcore extension module, that is maintained by Daisuke Hatayama,
> > and he make all decisions w/respect to that codebase. I've cc'd this response
> > to him.
>
> Thanks Zhao for your patch set.
> Thanks for ccing me, Dave.
>
> I agree that ZRAM support is useful as your explanation. On the other
> hand, it is not only for crash gcore command, but also for crash utility. I think
> it more natural than the current implementation of your patch set that you
> implement a ZRAM support in crash utility and then do it in crash gcore command.
>
> If the ZRAM support were transparent to readmem() interface, there would be no need
> to implement crash gcore command at all. If not, there would be need to add a new code
> for the ZRAM support just corresponding to the following stanza in
> 0001-gcore-add-support-zram-swap.patch:
>
> @@ -225,6 +417,18 @@ void gcore_coredump(void)
> strerror(errno));
> } else {
> pagefaultf("page fault at %lx\n", addr);
> + if (paddr != 0) {
> + pte_val = paddr;
> + if(try_zram_decompress(pte_val, (unsigned char *)buffer) == PAGE_SIZE)
> + {
> + error(WARNING, "zram decompress successed\n");
> + if (fwrite(buffer, PAGE_SIZE, 1, gcore->fp) != 1)
> + error(FATAL, "%s: write: %s\n", gcore->corename, strerror(errno));
> + continue;
> + }
> +
> + }
I'm not clear on how zram is linked into the user-space mapping. For user space that
has been swapped out to a zram swap device, I presume it's the same as is, but it
references the zram swap device. But for other user-space mappings (non-swapped),
what does the "vm -p" display for user space virtual address pages that are backed
by zram? And for that matter, what does "vtop <address>" show?
Thanks,
Dave
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