Initialize StoreMapSlice when reserving a new cache slot (#350)

Rock sets the StoreMapSlice::next field when sending a slice to disk. To
avoid writing slice A twice, Rock allocates a new slice B to prime
A.next right before writing A. Scheduling A's writing and, sometimes,
lack of data to fill B create a gap between B's allocation and B's
writing (which sets B.next). During that time, A.next points to B, but
B.next is untouched.

If writing slice A or swapout in general fails, the chain of failed
entry slices (now containing both A and B) is freed. If untouched B.next
contains garbage, then freeChainAt() adds "random" slices after B to the
free slice pool. Subsequent swapouts use those incorrectly freed slices,
effectively overwriting portions of random cache entries, corrupting the
cache.

How did B.next get dirty in the first place? freeChainAt() cleans the
slices it frees, but Rock also makes direct noteFreeMapSlice() calls.
Shared memory cache may have avoided this corruption because it makes no
such calls.

Ipc::StoreMap::prepFreeSlice() now clears allocated slices. Long-term,
we may be able to move free slice management into StoreMap to automate
this cleanup.

Also simplified and polished slot allocation code a little, removing the
Rock::IoState::reserveSlotForWriting() middleman. This change also
improves the symmetry between Rock and shared memory cache code.

diff --git a/src/MemStore.cc b/src/MemStore.cc
index a315f7adeffa9ab1f4df59fa0410a21d2bdb641e..af7624eb6c22ae57fbabea60b7b7ea8988c4b702 100644 (file)
--- a/src/MemStore.cc
+++ b/src/MemStore.cc
@@ -772,13 +772,15 @@ MemStore::reserveSapForWriting(Ipc::Mem::PageId &page)
 {
     Ipc::Mem::PageId slot;
     if (freeSlots->pop(slot)) {
-        debugs(20, 5, "got a previously free slot: " << slot);
+        const auto slotId = slot.number - 1;
+        debugs(20, 5, "got a previously free slot: " << slotId);
 
         if (Ipc::Mem::GetPage(Ipc::Mem::PageId::cachePage, page)) {
             debugs(20, 5, "and got a previously free page: " << page);
-            return slot.number - 1;
+            map->prepFreeSlice(slotId);
+            return slotId;
         } else {
-            debugs(20, 3, "but there is no free page, returning " << slot);
+            debugs(20, 3, "but there is no free page, returning " << slotId);
             freeSlots->push(slot);
         }
     }
@@ -791,8 +793,10 @@ MemStore::reserveSapForWriting(Ipc::Mem::PageId &page)
         assert(!waitingFor); // noteFreeMapSlice() should have cleared it
         assert(slot.set());
         assert(page.set());
-        debugs(20, 5, "got previously busy " << slot << " and " << page);
-        return slot.number - 1;
+        const auto slotId = slot.number - 1;
+        map->prepFreeSlice(slotId);
+        debugs(20, 5, "got previously busy " << slotId << " and " << page);
+        return slotId;
     }
     assert(waitingFor.slot == &slot && waitingFor.page == &page);
     waitingFor.slot = NULL;

diff --git a/src/fs/rock/RockIoState.cc b/src/fs/rock/RockIoState.cc
index bba24ae5c4cf5ad76a6932e843631dd49e8a78d0..50b6e6d66c9ee6c556353a138aca35522db34e96 100644 (file)
--- a/src/fs/rock/RockIoState.cc
+++ b/src/fs/rock/RockIoState.cc
@@ -190,7 +190,7 @@ Rock::IoState::tryWrite(char const *buf, size_t size, off_t coreOff)
     // allocate the first slice during the first write
     if (!coreOff) {
         assert(sidCurrent < 0);
-        sidCurrent = reserveSlotForWriting(); // throws on failures
+        sidCurrent = dir->reserveSlotForWriting(); // throws on failures
         assert(sidCurrent >= 0);
         writeAnchor().start = sidCurrent;
     }
@@ -207,7 +207,7 @@ Rock::IoState::tryWrite(char const *buf, size_t size, off_t coreOff)
         // We do not write a full buffer without overflow because
         // we would not yet know what to set the nextSlot to.
         if (overflow) {
-            const SlotId sidNext = reserveSlotForWriting(); // throws
+            const auto sidNext = dir->reserveSlotForWriting(); // throws
             assert(sidNext >= 0);
             writeToDisk(sidNext);
         } else if (Store::Root().transientReaders(*e)) {
@@ -306,21 +306,6 @@ Rock::IoState::writeBufToDisk(const SlotId sidNext, const bool eof, const bool l
     theFile->write(r);
 }
 
-/// finds and returns a free db slot to fill or throws
-Rock::SlotId
-Rock::IoState::reserveSlotForWriting()
-{
-    Ipc::Mem::PageId pageId;
-    if (dir->useFreeSlot(pageId))
-        return pageId.number-1;
-
-    // This may happen when the number of available db slots is close to the
-    // number of concurrent requests reading or writing those slots, which may
-    // happen when the db is "small" compared to the request traffic OR when we
-    // are rebuilding and have not loaded "many" entries or empty slots yet.
-    throw TexcHere("ran out of free db slots");
-}
-
 void
 Rock::IoState::finishedWriting(const int errFlag)
 {

diff --git a/src/fs/rock/RockIoState.h b/src/fs/rock/RockIoState.h
index 3c213558e63455c604acfa573cd681ad65f3e5c6..b8f2061e42e12060e36d77d0c0296cdfe4cddd54 100644 (file)
--- a/src/fs/rock/RockIoState.h
+++ b/src/fs/rock/RockIoState.h
@@ -66,7 +66,6 @@ private:
     size_t writeToBuffer(char const *buf, size_t size);
     void writeToDisk(const SlotId nextSlot);
     void writeBufToDisk(const SlotId nextSlot, const bool eof, const bool lastWrite);
-    SlotId reserveSlotForWriting();
 
     void callBack(int errflag);
 

diff --git a/src/fs/rock/RockSwapDir.cc b/src/fs/rock/RockSwapDir.cc
index 7eec16381ca47714f2685cc337758947cd6de68c..894c582c902489d2ec83ed301e40b34cb5fbbaf4 100644 (file)
--- a/src/fs/rock/RockSwapDir.cc
+++ b/src/fs/rock/RockSwapDir.cc
@@ -710,12 +710,16 @@ Rock::SwapDir::diskOffsetLimit() const
     return diskOffset(map->sliceLimit());
 }
 
-bool
-Rock::SwapDir::useFreeSlot(Ipc::Mem::PageId &pageId)
+Rock::SlotId
+Rock::SwapDir::reserveSlotForWriting()
 {
+    Ipc::Mem::PageId pageId;
+
     if (freeSlots->pop(pageId)) {
-        debugs(47, 5, "got a previously free slot: " << pageId);
-        return true;
+        const auto slotId = pageId.number - 1;
+        debugs(47, 5, "got a previously free slot: " << slotId);
+        map->prepFreeSlice(slotId);
+        return slotId;
     }
 
     // catch free slots delivered to noteFreeMapSlice()
@@ -724,14 +728,20 @@ Rock::SwapDir::useFreeSlot(Ipc::Mem::PageId &pageId)
     if (map->purgeOne()) {
         assert(!waitingForPage); // noteFreeMapSlice() should have cleared it
         assert(pageId.set());
-        debugs(47, 5, "got a previously busy slot: " << pageId);
-        return true;
+        const auto slotId = pageId.number - 1;
+        debugs(47, 5, "got a previously busy slot: " << slotId);
+        map->prepFreeSlice(slotId);
+        return slotId;
     }
     assert(waitingForPage == &pageId);
     waitingForPage = NULL;
 
+    // This may happen when the number of available db slots is close to the
+    // number of concurrent requests reading or writing those slots, which may
+    // happen when the db is "small" compared to the request traffic OR when we
+    // are rebuilding and have not loaded "many" entries or empty slots yet.
     debugs(47, 3, "cannot get a slot; entries: " << map->entryCount());
-    return false;
+    throw TexcHere("ran out of free db slots");
 }
 
 bool

diff --git a/src/fs/rock/RockSwapDir.h b/src/fs/rock/RockSwapDir.h
index 879bd45ed0393ef600e290e03fa0d8e82bbd859b..aa8828375829feb7be7ed103acd893e16b89a065 100644 (file)
--- a/src/fs/rock/RockSwapDir.h
+++ b/src/fs/rock/RockSwapDir.h
@@ -62,10 +62,12 @@ public:
     int64_t slotLimitAbsolute() const; ///< Rock store implementation limit
     int64_t slotLimitActual() const; ///< total number of slots in this db
 
-    /// removes a slot from a list of free slots or returns false
-    bool useFreeSlot(Ipc::Mem::PageId &pageId);
     /// whether the given slot ID may point to a slot in this db
     bool validSlotId(const SlotId slotId) const;
+
+    /// finds and returns a free db slot to fill or throws
+    SlotId reserveSlotForWriting();
+
     /// purges one or more entries to make full() false and free some slots
     void purgeSome();
 

diff --git a/src/ipc/StoreMap.cc b/src/ipc/StoreMap.cc
index 203b1804ef1f2bda00d740665217613e09785330..072e3fc014bf3a62fd40898efb74b379b8e10cc3 100644 (file)
--- a/src/ipc/StoreMap.cc
+++ b/src/ipc/StoreMap.cc
@@ -346,8 +346,7 @@ Ipc::StoreMap::freeChainAt(SliceId sliceId, const SliceId splicingPoint)
     while (sliceId >= 0) {
         Slice &slice = sliceAt(sliceId);
         const SliceId nextId = slice.next;
-        slice.size = 0;
-        slice.next = -1;
+        slice.clear();
         if (cleaner)
             cleaner->noteFreeMapSlice(sliceId); // might change slice state
         if (sliceId == splicingPoint) {
@@ -360,6 +359,14 @@ Ipc::StoreMap::freeChainAt(SliceId sliceId, const SliceId splicingPoint)
     debugs(54, 7, "freed chain #" << chainId << " in " << path);
 }
 
+void
+Ipc::StoreMap::prepFreeSlice(const SliceId sliceId)
+{
+    // TODO: Move freeSlots here, along with reserveSlotForWriting() logic.
+    assert(validSlice(sliceId));
+    sliceAt(sliceId).clear();
+}
+
 Ipc::StoreMap::SliceId
 Ipc::StoreMap::sliceContaining(const sfileno fileno, const uint64_t bytesNeeded) const
 {

diff --git a/src/ipc/StoreMap.h b/src/ipc/StoreMap.h
index 5d44379b61b1c673cd5a5fd719ceb11f0c31812e..8a4909e7796eccf5922fc40e08081cb11dba19fa 100644 (file)
--- a/src/ipc/StoreMap.h
+++ b/src/ipc/StoreMap.h
@@ -42,6 +42,9 @@ public:
         return *this;
     }
 
+    /// restore default-constructed state
+    void clear() { size = 0; next = -1; }
+
     std::atomic<Size> size; ///< slice contents size
     std::atomic<StoreMapSliceId> next; ///< ID of the next entry slice
 };
@@ -292,6 +295,9 @@ public:
     /// readable anchor for the entry created by openForReading()
     const Anchor &readableEntry(const AnchorId anchorId) const;
 
+    /// prepare a chain-unaffiliated slice for being added to an entry chain
+    void prepFreeSlice(const SliceId sliceId);
+
     /// Returns the ID of the entry slice containing n-th byte or
     /// a negative ID if the entry does not store that many bytes (yet).
     /// Requires a read lock.