Re: [PATCH v5 3.1.0-rc4-tip 5/26] Uprobes: copy of the originalinstruction.
From: Oleg Nesterov
Date: Wed Oct 05 2011 - 11:16:30 EST
I am going to discuss the things I do not really understand ;)
Hopefully someone will correct me if I am wrong.
On 10/05, Srikar Dronamraju wrote:
> * Oleg Nesterov <oleg@xxxxxxxxxx> [2011-10-03 18:29:05]:
> > > + page_cache_sync_readahead(mapping, &filp->f_ra, filp, idx, 1);
> > This schedules the i/o,
> > > + page = grab_cache_page(mapping, idx);
> > This finds/locks the page in the page-cache,
> > > + if (!page)
> > > + return -ENOMEM;
> > > +
> > > + vaddr = kmap_atomic(page);
> > > + memcpy(insn, vaddr + off1, nbytes);
> > What if this page is not PageUptodate() ?
> Since we do a synchronous read ahead, I thought the page would be
> populated and upto date.
What does this "synchronous" actually mean?
First of all, page_cache_sync_readahead() can simply return. Or
__do_page_cache_readahead() can "skip" the page if it is already in the
IOW, we do not even know if ->readpage() was called. But even if it was
called, afaics (in general) the page will be unlocked and marked Uptodate
when I/O completes, not when ->readpage() returns.
> would these two lines after grab_cache_page help?
> if (!PageUptodate(page))
> mapping->a_ops->readpage(filp, page);
This doesn't look right. At least you need lock_page().
Anyway. Why you can't simply use read_mapping_page() or even kernel_read() ?
But the real question is:
> > But I am starting to think I simply do not understand this change.
> > To the point, I do not underestand why do we need copy_insn() at all.
> > We are going to replace this page, can't we save/analyze ->insn later
> > when we copy the content of the old page? Most probably I missed
> > something simple...
Could you please explain?
> > But this should not be possible, no? How it can map this vaddr above
> > TASK_SIZE ?
> > get_user_pages(tsk => NULL) is fine. Why else do we need mm->owner ?
> > Probably used by the next patches... Say, is_32bit_app(tsk). This
> > can use mm->context.ia32_compat (hopefully will be replaced with
> > MMF_COMPAT).
> We used the tsk struct for checking if the application was 32 bit and
> for calling get_user_pages. Since we can pass NULL to get_user_pages and
> since we can use mm->context.ia32_compat or MMF_COMPAT, I will remove
> get_mm_owner, that way we dont need to be dependent on CONFIG_MM_OWNER.
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