Add getSlabMemoryInfo() for IO registration

Summary:
Expose CacheLib slab memory base address and size via public API. This allows IO operations (e.g., RDMA, io_uring) to pre-register the entire cache arena for zero-copy get/put operations, avoiding per-API overhead.

Returns the user-visible slab data region only, excluding internal slab headers at the start of the memory arena. This prevents IO from corrupting CacheLib metadata.

Reviewed By: AlnisM

Differential Revision: D110330084

fbshipit-source-id: 87811ebfb14f714b16689811de89efdaf30dcc97

Author: rlyerly

cachelib/allocator/CacheAllocator.h

@@ -1283,6 +1283,12 @@ class CacheAllocator : public CacheBase {
   // return cache's memory usage stats
   CacheMemoryStats getCacheMemoryStats() const override final;
 
+  // Return slab memory base address and size; useful for APIs that want to
+  // pre-register buffers to avoid IO overhead (e.g., mapping pages in kernel).
+  std::pair<const void*, size_t> getSlabMemoryInfo() const noexcept {
+    return allocator_->getSlabMemoryInfo();
+  }
+
   // return the nvm cache stats map
   util::StatsMap getNvmCacheStatsMap() const override final;
 

cachelib/allocator/memory/MemoryAllocator.h

@@ -499,6 +499,11 @@ class MemoryAllocator {
     return (slab && slabAllocator_.isValidSlab(slab));
   }
 
+  // Get slab memory base address and size
+  std::pair<const void*, size_t> getSlabMemoryInfo() const noexcept {
+    return slabAllocator_.getSlabMemoryInfo();
+  }
+
   // fetch the allocation size for the pool id and class id.
   //
   // @param pid  the pool id

cachelib/allocator/memory/SlabAllocator.h

@@ -363,6 +363,14 @@ class SlabAllocator {
 #endif
   }
 
+  // Returns the user-visible slab data region, excluding internal slab headers
+  // at the start of the memory arena.
+  std::pair<const void*, size_t> getSlabMemoryInfo() const noexcept {
+    auto slabStart = reinterpret_cast<uintptr_t>(slabMemoryStart_);
+    auto memStart = reinterpret_cast<uintptr_t>(memoryStart_);
+    return {slabMemoryStart_, memorySize_ - (slabStart - memStart)};
+  }
+
  private:
   // null Slab* presenttation. With 4M Slab size, a valid slab index would never
   // reach 2^16 - 1;