Re: [PATCH] zram: Using GFP_ATOMIC instead of GFP_KERNEL to allocate bitmap memory in backing_dev_store

[Sergey Senozhatsky]

On (23/12/01 07:19), Jens Axboe wrote:
> >> IOW, you have a slew of GFP_KERNEL allocations in there, and you
> >> probably just patched the largest one. But the core issue remains.
> >>
> >> The whole handling of backing_dev_store() looks pretty broken.
> >>
> > 
> > Indeed, this patch only solves the biggest problem and does not
> > fundamentally solve it, because there are many processes for holding
> > zram->init_lock before allocation memory in backing_dev_store that
> > need to be fully modified, and I did not consider it thoroughly.
> > Obviously, a larger and better patch is needed to eliminate this risk,
> > but it is currently not necessary.
> 
> You agree that it doesn't fix the issue, it just happens to fix the one
> that you hit. And then you jump to the conclusion that this is all
> that's needed to fix it. Ehm, confused?

Yeah.

zram is very sensitive to memory - zsmalloc pool needs physical pages
(allocated on demand) to keep data written to zram. zram probably simply
should not be used on systems under such heavy memory pressure. Throwing
GPF_ATOMICs at zram isn't going to fix anything.

Jens, you said that zram's backing device handling is broken. Got a minute
to elaborate?