Re: [PATCH] [13/16] HWPOISON: The high level memory error handler in the VM v3

[Nai Xia]

On Mon, Jun 8, 2009 at 8:31 PM, Wu Fengguang<fengguang.wu@xxxxxxxxx> wrote:
> On Mon, Jun 08, 2009 at 07:06:12PM +0800, Nai Xia wrote:
>> On Mon, Jun 8, 2009 at 12:02 AM, Wu Fengguang<fengguang.wu@xxxxxxxxx> wrote:
>> > On Thu, Jun 04, 2009 at 02:25:24PM +0800, Nai Xia wrote:
>> >> On Thu, May 28, 2009 at 10:50 PM, Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
>> >> > On Thu, May 28, 2009 at 09:45:20PM +0800, Andi Kleen wrote:
>> >> >> On Thu, May 28, 2009 at 02:08:54PM +0200, Nick Piggin wrote:
>> >> >
>> >> > [snip]
>> >> >
>> >> >> >
>> >> >> > BTW. I don't know if you are checking for PG_writeback often enough?
>> >> >> > You can't remove a PG_writeback page from pagecache. The normal
>> >> >> > pattern is lock_page(page); wait_on_page_writeback(page); which I
>> >> >>
>> >> >> So pages can be in writeback without being locked? I still
>> >> >> wasn't able to find such a case (in fact unless I'm misreading
>> >> >> the code badly the writeback bit is only used by NFS and a few
>> >> >> obscure cases)
>> >> >
>> >> > Yes the writeback page is typically not locked. Only read IO requires
>> >> > to be exclusive. Read IO is in fact page *writer*, while writeback IO
>> >> > is page *reader* :-)
>> >>
>> >> Sorry for maybe somewhat a little bit off topic,
>> >> I am trying to get a good understanding of PG_writeback & PG_locked ;)
>> >>
>> >> So you are saying PG_writeback & PG_locked are acting like a read/write lock?
>> >> I notice wait_on_page_writeback(page) seems always called with page locked --
>> >
>> > No. Note that pages are not locked in wait_on_page_writeback_range().
>>
>> I see. This function seems mostly called  on the sync path,
>> it just waits for data being synchronized to disk.
>> No writers from the pages' POV, so no lock.
>> I missed this case, but my argument about the role of read/write lock.
>> seems still consistent. :)
>
> It's more constrained. Normal read/write locks allow concurrent readers,
> however fsync() must wait for previous IO to finish before starting
> its own IO.

Oh, yes, this is what I called "mixed roles". One for lock, one for
status flag, twisted together in the same path, making the read lock
semantics totally broken.

>
>> >
>> >> that is the semantics of a writer waiting to get the lock while it's
>> >> acquired by
>> >> some reader:The caller(e.g. truncate_inode_pages_range()  and
>> >> invalidate_inode_pages2_range()) are the writers waiting for
>> >> writeback readers (as you clarified ) to finish their job, right ?
>> >
>> > Sorry if my metaphor confused you. But they are not typical
>> > reader/writer problems, but more about data integrities.
>>
>> No, you didn't :)
>> Actually, you make me clear about the mixed roles for
>> those bits.
>>
>> >
>> > Pages have to be "not under writeback" when truncated.
>> > Otherwise data lost is possible:
>> >
>> > 1) create a file with one page (page A)
>> > 2) truncate page A that is under writeback
>> > 3) write to file, which creates page B
>> > 4) sync file, which sends page B to disk quickly
>> >
>> > Now if page B reaches disk before A, the new data will be overwritten
>> > by truncated old data, which corrupts the file.
>>
>> I fully understand this scenario which you had already clarified in a
>> previous message. :)
>>
>> 1. someone make index1-> page A
>> 2. Path P1 is acting as a *reader* to a cache page at index1 by
>>     setting PG_writeback on, while at the same time as a *writer* to
>>     the corresponding file blocks.
>> 3. Another path P2 comes in and  truncate page A, he is the writer
>>     to the same cache page.
>> 4. Yet another path P3 comes  as the writer to the cache page
>>      making it points to page B: index1--> page B.
>> 5. Path P4 comes writing back the cache page(and set PG_writeback).
>>    He is the reader of the cache page and the writer to the file blocks.
>>
>> The corrupts occur because P1 & P4 races when writing file blocks.
>> But the _root_ of this racing is because nothing is used to serialize
>> them on the side of writing the file blocks and above stream reading was
>> inconsistent because of the writers(P2 & P3) to cache page at index1.
>>
>> Note that the "sync file" is somewhat irrelevant, even without "sync file",
>> the racing still may exists. I know you must want to show me that this could
>> make the corruption more easy to occur.
>>
>> So I think the simple logic is:
>> 1) if you want to truncate/change the mapping from a index to a struct *page,
>> test writeback bit because the writebacker to the file blocks is the reader
>> of this mapping.
>> 2) if a writebacker want to start a read of this mapping with
>> test_set_page_writeback()
>> or set_page_writeback(), he'd be sure this page is locked to keep out the
>> writers to this mapping of index-->struct *page.
>>
>> This is really behavior of a read/write lock, right ?
>
> Please, that's a dangerous idea. A page can be written to at any time
> when writeback to disk is under way. Does PG_writeback (your reader
> lock) prevents page data writers?  NO.

I meant PG_writeback stops writers to index---->struct page mapping.

I think I should make my statements more concise and the "reader/writer"
less vague.

Here we care about the write/read operation for index---->struct page mapping.
Not for read/write operation for the page content.

Anyone who wants to change this mapping  is a writer, he should take
page lock.
Anyone who wants to reference this mapping is a reader, writers should
wait for him. And when this reader wants to get ref, he should wait for
anyone one who is changing this mapping(e.g. page truncater).

When a path sets PG_writeback on a page, it need this index-->struct page
mapping be 100% valid right? (otherwise may leads to corruption.)
So writeback routines are readers of this index-->struct page mapping.
(oh, well if we can put the other role of PG_writeback aside)

Ok,Ok, since PG_locked does mean much more than just protecting
the per-page mapping which makes the lock abstraction even less clear.
so indeed, forget about it.

>
> Thanks,
> Fengguang
>
>> wait_on_page_writeback_range() looks different only because "sync"
>> operates on "struct page", it's not sensitive to index-->struct *page mapping.
>> It does care about if pages returned by pagevec_lookup_tag() are
>> still maintains the mapping when wait_on_page_writeback(page).
>> Here, PG_writeback is only a status flag for "struct page" not a lock bit for
>> index->struct *page mapping.
>>
>> >
>> >> So do you think the idea is sane to group the two bits together
>> >> to form a real read/write lock, which does not care about the _number_
>> >> of readers ?
>> >
>> > We don't care number of readers here. So please forget about it.
>> Yeah, I meant number of readers is not important.
>>
>> I still hold that these two bits in some way act like a _sparse_
>> read/write lock.
>> But I am going to drop the idea of making them a pure lock, since PG_writeback
>> does has other meaning -- the page is being writing back: for sync
>> path, it's only
>> a status flag.
>> Making a pure read/write lock definitely will lose that or at least distort it.
>>
>>
>> Hoping I've made my words understandable, correct me if wrong, and
>> many thanks for your time and patience. :-)
>>
>>
>> Nai Xia
>>
>> >
>> > Thanks,
>> > Fengguang
>> >
>> >> > The writeback bit is _widely_ used.  test_set_page_writeback() is
>> >> > directly used by NFS/AFS etc. But its main user is in fact
>> >> > set_page_writeback(), which is called in 26 places.
>> >> >
>> >> >> > think would be safest
>> >> >>
>> >> >> Okay. I'll just add it after the page lock.
>> >> >>
>> >> >> > (then you never have to bother with the writeback bit again)
>> >> >>
>> >> >> Until Fengguang does something fancy with it.
>> >> >
>> >> > Yes I'm going to do it without wait_on_page_writeback().
>> >> >
>> >> > The reason truncate_inode_pages_range() has to wait on writeback page
>> >> > is to ensure data integrity. Otherwise if there comes two events:
>> >> >        truncate page A at offset X
>> >> >        populate page B at offset X
>> >> > If A and B are all writeback pages, then B can hit disk first and then
>> >> > be overwritten by A. Which corrupts the data at offset X from user's POV.
>> >> >
>> >> > But for hwpoison, there are no such worries. If A is poisoned, we do
>> >> > our best to isolate it as well as intercepting its IO. If the interception
>> >> > fails, it will trigger another machine check before hitting the disk.
>> >> >
>> >> > After all, poisoned A means the data at offset X is already corrupted.
>> >> > It doesn't matter if there comes another B page.
>> >> >
>> >> > Thanks,
>> >> > Fengguang
>> >> > --
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>> >> >
>> >
>
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