Re: [PATCH v2] staging: zcache: support multiple clients, prep for KVM and RAMster

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

On 08/10/2011 09:21 AM, Seth Jennings wrote:
> On 06/30/2011 02:01 PM, Dan Magenheimer wrote:
>> Hi Greg --
>>
>> I think this patch is now ready for staging-next and for merging when
>> the 3.1 window opens.  Please let me know if you need any logistics
>> done differently.
>>
>> Thanks,
>> Dan
>>
>> ===
>>
>>> From: Dan Magenheimer <dan.magenheimer@oracle.com>
>> Subject: staging: zcache: support multiple clients, prep for KVM and RAMster
>>
>> This is version 2 of an update to zcache, incorporating feedback from the list.
>> This patch adds support to the in-kernel transcendent memory ("tmem") code
>> and the zcache driver for multiple clients, which will be needed for both
>> RAMster and KVM support.  It also adds additional tmem callbacks to support
>> RAMster and corresponding no-op stubs in the zcache driver.  In v2, I've
>> also taken the liberty of adding some additional sysfs variables to
>> both surface information and allow policy control.  Those experimenting
>> with zcache should find them useful.
>>
>> Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
>>
>> [v2: konrad.wilk@oracle.com: fix bools, add check for NULL, fix a comment]
>> [v2: sjenning@linux.vnet.ibm.com: add info/tunables for poor compression]
>> [v2: marcusklemm@googlemail.com: add tunable for max persistent pages]
>> Cc: Nitin Gupta <ngupta@vflare.org>
>> Cc: linux-mm@kvack.org
>> Cc: kvm@vger.kernel.org
>>
>> ===
>>
>> Diffstat:
>>  drivers/staging/zcache/tmem.c            |  100 +++-
>>  drivers/staging/zcache/tmem.h            |   23 
>>  drivers/staging/zcache/zcache.c          |  512 +++++++++++++++++----
>>  3 files changed, 520 insertions(+), 115 deletions(-)
>>
>>
> <cut>
>> @@ -901,48 +1144,59 @@ static unsigned long zcache_curr_pers_pa
>>  /* forward reference */
>>  static int zcache_compress(struct page *from, void **out_va, size_t *out_len);
>>  
>> -static void *zcache_pampd_create(struct tmem_pool *pool, struct tmem_oid *oid,
>> -				 uint32_t index, struct page *page)
>> +static void *zcache_pampd_create(char *data, size_t size, bool raw, int eph,
>> +				struct tmem_pool *pool, struct tmem_oid *oid,
>> +				 uint32_t index)
>>  {
>>  	void *pampd = NULL, *cdata;
>>  	size_t clen;
>>  	int ret;
>> -	bool ephemeral = is_ephemeral(pool);
>>  	unsigned long count;
>> +	struct page *page = virt_to_page(data);
> 
> With zcache_put_page() modified to pass page_address(page) instead of the 
> actual page structure, in combination with the function signature changes 
> to tmem_put() and zcache_pampd_create(), zcache_pampd_create() tries to 
> (re)derive the page structure from the virtual address.  However, if the 
> original page is a high memory page (or any unmapped page), this 
> virt_to_page() fails because the page_address() in zcache_put_page()
> returned NULL.
> 
> With CONFIG_DEBUG_VIRTUAL set, the BUG message is this:
> ==========
> [  101.347711] ------------[ cut here ]------------
> [  101.348030] kernel BUG at arch/x86/mm/physaddr.c:51!
> [  101.348030] invalid opcode: 0000 [#1] DEBUG_PAGEALLOC
> [  101.348030] Modules linked in:
> [  101.348030] 
> [  101.348030] Pid: 20, comm: kswapd0 Not tainted 3.1.0-rc1+ #229 Bochs Bochs
> [  101.348030] EIP: 0060:[<c1058c9a>] EFLAGS: 00010013 CPU: 0
> [  101.348030] EIP is at __phys_addr+0x1a/0x50
> [  101.348030] EAX: 00000000 EBX: f5e64000 ECX: 00000000 EDX: 00001000
> [  101.348030] ESI: f6ffd000 EDI: 00000000 EBP: f6353c10 ESP: f6353c10
> [  101.348030]  DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
> [  101.348030] Process kswapd0 (pid: 20, ti=f6352000 task=f69b9c20 task.ti=f6352000)
> [  101.348030] Stack:
> [  101.348030]  f6353c60 c12e4114 00000000 00000001 00000000 c12e5682 00000000 00000046
> [  101.348030]  00000000 f5e65668 f5e65658 f5e65654 f580f000 f6353c60 c13904f5 00000000
> [  101.348030]  f5e65654 f5e64000 f5eab000 00000000 f6353cb0 c12e5713 00000000 f5e64000
> [  101.348030] Call Trace:
> [  101.348030]  [<c12e4114>] zcache_pampd_create+0x14/0x6a0
> [  101.348030]  [<c12e5682>] ? tmem_put+0x52/0x3f0
> [  101.348030]  [<c13904f5>] ? _raw_spin_lock+0x45/0x50
> [  101.348030]  [<c12e5713>] tmem_put+0xe3/0x3f0
> [  101.348030]  [<c10f5358>] ? page_address+0xb8/0xe0
> [  101.348030]  [<c12e498b>] zcache_frontswap_put_page+0x1eb/0x2e0
> [  101.348030]  [<c110798b>] __frontswap_put_page+0x6b/0x110
> [  101.348030]  [<c1103ccf>] swap_writepage+0x8f/0xf0
> [  101.348030]  [<c10ee377>] shmem_writepage+0x1a7/0x1d0
> [  101.348030]  [<c10ea907>] shrink_page_list+0x3f7/0x7c0
> [  101.348030]  [<c10eafdd>] shrink_inactive_list+0x12d/0x360
> [  101.348030]  [<c10eb5f8>] shrink_zone+0x3e8/0x530
> [  101.348030]  [<c10ebc92>] kswapd+0x552/0x940
> [  101.348030]  [<c1084460>] ? wake_up_bit+0x30/0x30
> [  101.348030]  [<c10eb740>] ? shrink_zone+0x530/0x530
> [  101.348030]  [<c10840e4>] kthread+0x74/0x80
> [  101.348030]  [<c1084070>] ? __init_kthread_worker+0x60/0x60
> [  101.348030]  [<c1397736>] kernel_thread_helper+0x6/0xd
> [  101.348030] Code: 00 c0 ff 81 eb 00 20 00 00 39 d8 72 cd eb ae 66 90 55 3d ff ff ff bf 89 e5 76 10 80 3d 10 88 53 c1 00 75 09 05 00 00 00 40 5d c3 <0f> 0b 8b 15 b0 6a 9e c1 81 c2 00 00 80 00 39 d0 72 e7 8b 15 a8 
> [  101.348030] EIP: [<c1058c9a>] __phys_addr+0x1a/0x50 SS:ESP 0068:f6353c10
> ==========
> 
> This crash is hit every time a high memory page is swapped out.
> 
> I have no solution right now other that to revert this patch and 
> restore the original signatures.
> 

Sorry for the noise, but I noticed right after I sent this that
the tmem layer doesn't DO anything with the data parameter. So
a possible solution is to just pass the page pointer instead of
the virtual address.  After all, pointers are pointers.

--- a/drivers/staging/zcache/zcache.c
+++ b/drivers/staging/zcache/zcache.c
@@ -1153,7 +1153,7 @@ static void *zcache_pampd_create(char *data, size_t size, 
        size_t clen;
        int ret;
        unsigned long count;
-       struct page *page = virt_to_page(data);
+       struct page *page = (struct page *)(data);
        struct zcache_client *cli = pool->client;
        uint16_t client_id = get_client_id_from_client(cli);
        unsigned long zv_mean_zsize;
@@ -1220,7 +1220,7 @@ static int zcache_pampd_get_data(char *data, size_t *bufsi
        int ret = 0;
 
        BUG_ON(is_ephemeral(pool));
-       zv_decompress(virt_to_page(data), pampd);
+       zv_decompress((struct page *)(data), pampd);
        return ret;
 }
 
@@ -1532,7 +1532,7 @@ static int zcache_put_page(int cli_id, int pool_id, struct
                goto out;
        if (!zcache_freeze && zcache_do_preload(pool) == 0) {
                /* preload does preempt_disable on success */
-               ret = tmem_put(pool, oidp, index, page_address(page),
+               ret = tmem_put(pool, oidp, index, (char *)(page),
                                PAGE_SIZE, 0, is_ephemeral(pool));
                if (ret < 0) {
                        if (is_ephemeral(pool))
@@ -1565,7 +1565,7 @@ static int zcache_get_page(int cli_id, int pool_id, struct
        pool = zcache_get_pool_by_id(cli_id, pool_id);
        if (likely(pool != NULL)) {
                if (atomic_read(&pool->obj_count) > 0)
-                       ret = tmem_get(pool, oidp, index, page_address(page),
+                       ret = tmem_get(pool, oidp, index, (char *)(page),
                                        &size, 0, is_ephemeral(pool));
                zcache_put_pool(pool);
        }

I tested this and it works.

Dan, does this mess anything else up?

> What was the rationale for the signature changes?
> 
> --
> Seth
> 
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