Re: [PATCH 5/8] vmalloc: simplify vread()/vwrite()

From: Wu Fengguang
Date: Thu Jan 21 2010 - 00:05:37 EST

On Mon, Jan 18, 2010 at 07:23:43PM -0700, KAMEZAWA Hiroyuki wrote:
> On Tue, 19 Jan 2010 09:33:03 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> > > The whole thing looks stupid though, apparently kmap is used to avoid "the
> > > lock". But the lock is already held. We should just use the vmap
> > > address.
> >
> > Yes. I wonder why Kame introduced kmap_atomic() in d0107eb07 -- given
> > that he at the same time fixed the order of removing vm_struct and
> > vmap in dd32c279983b.
> >
> Hmm...I must check my thinking again before answering..
> vmalloc/vmap is constructed by 2 layer.
> - vmalloc layer....guarded by vmlist_lock.
> - vmap layer ....gurderd by purge_lock. etc.
> Now, let's see how vmalloc() works. It does job in 2 steps.
> vmalloc():
> - allocate vmalloc area to the list under vmlist_lock.
> - map pages.
> vfree()
> - free vmalloc area from the list under vmlist_lock.
> - unmap pages under purge_lock.
> Now. vread(), vwrite() just take vmlist_lock, doesn't take purge_lock().
> It walks page table and find pte entry, page, kmap and access it.
> Oh, yes. It seems it's safe without kmap. But My concern is percpu allocator.
> It uses get_vm_area() and controls mapped pages by themselves and
> map/unmap pages by with their own logic. vmalloc.c is just used for
> alloc/free virtual address.
> Now, vread()/vwrite() just holds vmlist_lock() and walk page table
> without no guarantee that the found page is stably mapped. So, I used kmap.
> If I miss something, I'm very sorry to add such kmap.

Ah Thanks for explanation!

I did some audit and find that

- set_memory_uc(), set_memory_array_uc(), set_pages_uc(),
set_pages_array_uc() are called EFI code and various video drivers,
all of them don't touch HIGHMEM RAM

- Kame: ioremap() won't allow remap of physical RAM

So kmap_atomic() is safe. Let's just settle on this patch?

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